Membrane 3D IC fabrication

ABSTRACT

General purpose methods for the fabrication of integrated circuits from flexible membranes formed of very thin low stress dielectric materials, such as silicon dioxide or silicon nitride, and semiconductor layers. Semiconductor devices are formed in a semiconductor layer of the membrane. The semiconductor membrane layer is initially formed from a substrate of standard thickness, and all but a thin surface layer of the substrate is then etched or polished away. In another version, the flexible membrane is used as support and electrical interconnect for conventional integrated circuit die bonded thereto, with the interconnect formed in multiple layers in the membrane. Multiple die can be connected to one such membrane, which is then packaged as a multi-chip module. Other applications are based on (circuit) membrane processing for bipolar and MOSFET transistor fabrication, low impedance conductor interconnecting fabrication, flat panel displays, maskless (direct write) lithography, and 3D IC fabrication.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of commonly assigned U.S. patentapplication Ser. No. 09/775,598, filed Feb. 5, 2001, now U.S. Pat. No.6,765,279, which is a continuation of U.S. patent application Ser. No.09/027,959, filed Feb. 23, 1998, now abandoned, which is a division ofU.S. patent application Ser. No. 08/850,749, filed May 2, 1997, now U.S.Pat. No. 5,985,693, which is a continuation of U.S. patent applicationSer. No. 08/315,905, filed Sep. 30, 1994, now U.S. Pat. No. 5,869,354,which is a division of U.S. patent application Ser. No. 07/865,412,filed Apr. 8, 1992, now U.S. Pat. No. 5,354,695, all of which areincorporated by reference herein in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods for fabricating integrated circuits onand in flexible membranes, and to structures fabricated using suchmethods.

2. Description of Related Art

Mechanically and thermally durable free standing dielectric andsemiconductor membranes have been disclosed with thicknesses of lessthan 2 μm. (See commonly invented U.S. Pat. No. 4,924,589, and U.S.patent application Ser. No. 07/482,135, filed Feb. 16, 1990, now U.S.Pat. No. 5,103,557, both incorporated herein by reference). Thisdisclosure combines the novel use of these technologies and otherintegrated circuit (IC) processing techniques to form ICs as membranestypically less than 8 μm thick. This approach to IC fabrication fallsunder the generic industry-established title known as DielectricIsolation (DI), and is inclusive of subject areas such asSilicon-on-Insulator (SOI) and Silicon-on-Sapphire (SOS). ICs formedfrom dielectric and semiconductor membranes can reduce significantly thenumber and complexity of processing steps presently used to providecomplete IC device isolation; dielectric isolation techniques thatprovide dielectric isolation on all surfaces of the individual circuitdevices comprising the complete IC are not as yet widely used in volumeIC fabrication. Integrated Circuits are defined as commonly understoodtoday when referring to SSI, MSI, LSI, VLSI, ULSI, etc. levels ofcircuit complexity.

SUMMARY OF THE INVENTION

This invention is directed to a general method for the fabrication ofintegrated circuits and interconnect metallization structures frommembranes of dielectric and semiconductor materials. The fabricationtechnology in accordance with this invention is referred to herein asMembrane Dielectric Isolation (MDI), and the circuits made from it ascircuit membranes. The novel use of materials. and processing techniquesprovides for the fabrication of high temperature, mechanically durable,large area free standing membranes (greater than 1 square cm in area)from low stress dielectric and/or semiconductor films. These membranespermit the application (continued use) of most of the establishedintegrated processing methods for the fabrication of circuit devices andinterconnect metallization.

In accordance with the invention, an integrated circuit is formed on atensile low stress dielectric membrane comprised of one layer or apartial layer of semiconductor material in which are formed circuitdevices and several layers of dielectric and interconnect metallization.Also, a structure in accordance with the invention is a tensile membraneof semiconductor material in which are formed circuit devices withmultiple layers of tensile low stress dielectric and metallizationinterconnect on either side of the semiconductor membrane.

The membrane structure is a processing or manufacturing structure forenabling the manufacture of novel and more cost effective integratedcircuits. This is in addition to an objective to manufacture anintegrated circuit, or portion thereof, in a membrane or thin film form.

The general categories of circuit membranes that can be made by thisinvention are:

1. Large scale dielectric isolated integrated circuits formed on or fromsemiconductor or non-semiconductor substrates.

2. Multi-layer interconnect metallization circuits formed on or fromsemiconductor or non-semiconductor substrates.

The primary objectives of the MDI fabrication technology disclosedherein are the cost effective manufacture of high performance, highdensity integrated circuits and integrated circuit interconnect with theelimination or reduction of detrimental electrical effects on theoperation of individual circuit devices (e.g. diodes, transistors, etc.)by completely isolating with a dielectric material each such circuitdevice from the common substrate upon which they are initiallyfabricated, and therefore, from each other, and to provide a moreversatile and efficient physical form factor for the application ofintegrate circuits. Some of the benefits of the MDI IC fabricationprocess are the elimination or reduction of substrate current leakage,capacitive coupling and parasitic transistor effects between adjoiningcircuit devices. The MDI IC fabrication process benefits extend toseveral other categories of IC fabrication such as lower IC processingcosts due to fewer IC isolation processing steps, greater IC transistordensities through the capability to use established IC processingtechniques to fabricate interconnect metallization on both sides of aMDI IC circuit membrane, and greater IC performance through noveltransistor structures.

The strength of the MDI processes is primarily drawn from two areas:

(1) The ability to make a large area flexible thin film free standingdielectric membrane, typically framed or suspended or constrained at itsedges by a substrate frame or ring, or bonded frame or ring. Thismembrane is able to withstand a wide range of IC processing techniquesand processing temperatures (of at least 400° C.) without noticeabledeficiency in performance. The present dielectric materials that meetthese requirements are silicon dioxide and silicon nitride films whenprepared with specific low stress film deposition recipes for instanceon equipment supplied by Novellus Systems, Inc. Dielectric free standingfilms created by CVD process methods such as silicon carbide, boronnitride, boron carbon nitride aluminum oxide, aluminum nitride, tantalumpentoxide, germanium nitride, calcium fluoride, and diamond have beenproduced, and can potentially be used as one of the dielectric materialsin a MDI circuit membrane when deposited at an appropriate level ofsurface stress. Advances in the technology for making low stressdielectric films will likely produce additional free standing films thatcan be used as described herein.

(2) The ability to form a uniform thin film single crystal semiconductorsubstrate either as the primary substrate of semiconductor devices or asa carrier substrate upon which semiconductor devices could be grownepitaxially. Several methods toward this end are disclosed herein, andother techniques which are modifications thereof exist. Further, incertain applications polycrystalline semiconductor membranes such. aspolysilicon can be used in substitution for monocrystalline material.

It is the combination of the use of low stress free standing dielectricfilms with the-appropriate processing qualities and membrane or thinfilm single crystalline (monocrystalline), polycrystalline or amorphoussemiconductor substrate formation that provides much of the advantage ofthe MDI IC fabrication process. The following methods are encompassedwithin the present disclosure:

-   -   1. Methods for the fabrication of low stress free standing (thin        film) dielectric membranes that encapsulate each semiconductor        device that comprises an IC.    -   2. Methods for the formation of uniform thickness semiconductor        membrane (thin film) substrates for use in combination with low        stress dielectric materials.    -   3. Methods for the fabrication of semiconductor devices within        and on a dielectric membrane that comprises a circuit membrane.    -   4. Methods for the formation of interconnect metallization        structures within and on a dielectric membrane that comprises a        circuit membrane.

The MDI circuit fabrication process in one embodiment starts with asemiconductor wafer substrate, and results in an IC in the form of acircuit membrane where each transistor or semiconductor device (SD) inthe IC has complete dielectric isolation from every other suchsemiconductor device in the IC. Only interconnect at the specificelectrode contact sites of the semiconductor devices provides electricalcontinuity between the semiconductor devices. The primary feature of theMDI process is complete electrical isolation of all semiconductordevices of an IC from all of the intervening semiconductor substrate onwhich or in which they were initially formed and to do so at lower costand process complexity than existing bulk IC processing methods. Otherfeatures of the MDI process are vertical electrode contact (backsideinterconnect metallization), confined lateral selective epitaxialgrowth, non-symmetric dopant profiles, and the use of a MDI circuitmembrane to serve as a conformal or projection mask for lithographyprocessing. Even if the initial substrate with which MDI processingbegins with is the most commonly used semiconductor silicon, theresulting IC need not be composed of silicon-based devices, but could beof any semiconductor device material such as GaAs, InP, HgCdTe, InSb ora combination of technologies such as silicon and GaAs grown on asilicon substrate through epitaxial means. Silicon is an inexpensive andwell understood semiconductor substrate material with superiormechanical handling properties relative to most other presentlyestablished semiconductor materials. The MDI process is not limited tostarting with a silicon substrate and the process definition of MDI isnot dependent on use of silicon; however, there are presently clearadvantages to using silicon as a starting semiconductor substrate, andthe chief embodiment disclosed herein of MDI uses a startingsemiconductor substrate material of silicon.

The benefits to fabricating an IC with the MDI process are significantover prior art methods, some of these benefits being:

-   -   1. Complete electrical isolation of semiconductor devices.    -   2. Vertical semiconductor device structures.    -   3. Lower processing costs through lower processing complexity or        fewer device isolation processing steps.    -   4. Conformal mask lithography through the membrane. substrate.    -   5. Control of depth of focus during lithography exposure due to        control of substrate thickness.    -   6. Application of interconnect metallization to both sides of        the IC.    -   7. Through-membrane (substrate) interconnect metallization        routing.    -   8. Three dimensional IC structures through the bonding of        circuit membrane IC layers.    -   9. Efficient conductive or radiant cooling of IC components of        circuit membrane.    -   10. Direct optical (laser) based communication between parallel        positioned membrane ICs.    -   11. Higher performance ICs.    -   12. Vertical semiconductor device structure formation.    -   13. Novel selective epitaxial device formation.

In some semiconductor technologies it is not necessary to have completeisolation between each transistor or semiconductor device, such ascertain applications using polycrystalline or amorphous TFTs (thin filmtransistors). This is not a limitation on the MDI process, becausesemiconductor device side wall isolation is an option in the MDIprocess. What is novel is that the MDI process provides general methodsby which thin films or membranes of dielectric and semiconductormaterials can be formed into a free standing IC or circuit membrane.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1 a to 1 j show a dielectric and semiconductor membrane substratein cross-section.

FIG. 2 shows an etched silicon substrate membrane in cross-section.

FIGS. 3 a, 3 b show dielectric membranes with semiconductor devices.

FIG. 4 shows an alignment mark of a circuit membrane in cross-section.

FIG. 5 shows support structures for a membrane structure isolationstructure.

FIG. 6 a to 6 i show a circuit membrane Air Tunnel structure.

FIG. 7 shows stacked circuit membranes with optical input/output.

FIG. 8 shows a three dimensional circuit membrane.

FIGS. 9 a to 9 j show fabrication of a MOSFET in a membrane.

FIGS. 10 a to 10 d show fabrication of a transistor by lateral epitaxialgrowth on a membrane.

FIGS. 11 a to 11 f show vertical MOSFET and bipolar transistors formedon a membrane.

FIG. 12 a to 12 g show transistor fabrication on a membrane usingconfined laterally doped epitaxy.

FIGS. 12 h to 12 j show cross-sections of selective epitaxial growth ona membrane.

FIGS. 13 a to 13 d show cross-sections of multi-chip modules.

FIG. 14 shows a cross-section of a membrane formed on a reusablesubstrate.

FIG. 15 shows a cross-section of the membrane of FIG. 14 with a supportframe attached.

FIGS. 16 a, 16 b show multi-chip nodules in packages.

FIGS. 17 a to 17 c show soldering of bond pads of a circuit membrane toa die.

FIG. 18 shows bond pads on a die.

FIGS. 19 a, 19 b show bonding and de-bonding of a die to a circuitmembrane.

FIGS. 20, 21 show two sides of a circuit membrane.

FIGS. 22 a to 22 c show formation of a metal trace in a circuit membraneby a lift-off process.

FIGS. 23 a, 23 b show use of a buried etch stop layer to form a circuitmembrane having a thinner inner portion.

FIGS. 24, 25 show a source-integrated light valve 30 for direct writelithography.

FIGS. 26, 27 are cross-sections of X-ray sources for the device of FIGS.24, 25.

FIGS. 28 a to 28 b show a coil for the device of FIG. 24.

FIGS. 29 a to 29 k show portions of a source-external radiation valvefor direct write lithography device.

FIGS. 29 l to 29 n and 29 p show use of fixed freestanding membranelithography masks.

FIG. 30 shows a cross-section of a lithographic 5 tool.

FIGS. 31 a to 31 c show cross-sections of a display formed on amembrane.

FIGS. 32 a, 32 b show bonding of two circuit membranes.

FIGS. 32 c and 32 d show die cut from a circuit membrane and bonded ontoa rigid substrate.

DETAILED DESCRIPTION OF THE INVENTION

The MDI process is the formation of an IC or interconnect metallizationcircuit as a free standing dielectric and/or semiconductor circuitmembrane. Each semiconductor device comprising an IC circuit membrane isa semiconductor device optionally isolated from adjoining semiconductordevices, and where each semiconductor device is formed on or in amembrane of semiconductor material typically less than 8 μm inthickness. The overall thickness of a circuit membrane is typically lessthan 50 μm and preferably less than 8 μm. The dielectric membrane iscompatible with most higher temperature IC processing techniques.

MDI Fabrication Process

Several process variations can be used to form the thin film or membraneof semiconductor material for use in the MDI process. Additional relatedmethods for forming semiconductor membranes may exist or come intoexistence and are included in the MDI technology.

Examples of some of the methods that can be used for forming siliconsingle crystal thin films are:

1. Heavily boron doped (typically greater than 10¹⁸ atoms/cm²) etch stoplayer (formed by diffusion, implant or epitaxy) with optional epitaxialSiGe (less than 20% Ge) anti-autodoping overlayer layer and optionalepitaxial layer.

2. O₂ (oxide) and N₂ (Nitride) implant etch stop barrier layer. Implantconcentrations are typically between 10 to 100 times less for formationof an etch stop barrier layer than that required to form a buried oxideor nitride dielectric isolation layer as presently done with a standardthickness silicon substrate.

3. Buried oxide etch stop barrier layer formed from a porous siliconlayer.

4. High precision double sided polished substrate and masked timedchemical etch back of back-side.

5. Electro-chemical etch stop.

6. Buried etch stop layer formation through anodic or thermal waferbonding in combination with precision substrate polishing and chemicaletching.

There are many established methods for forming thin semiconductorsubstrates or membranes. The MDI process requires that the semiconductormembrane forming process (thinning process) produce a highly uniformmembrane typically less than 2 μm thick and that the surface tension ofthe semiconductor membrane be in low tensile stress. If the membrane isnot in tensile stress, but in compressive stress, surface flatness andmembrane structural integrity will in many cases be inadequate forsubsequent device fabrication steps or the ability to form asufficiently durable free standing membrane.

The use of highly doped layers on the surface or near the surface of thesubstrate formed by diffusion, implant or epitaxial means is anestablished method for forming a barrier etch stop layer. A heavilydoped boron layer will etch 10 to 100 times slower than the rest of thesubstrate. However, if it is to form an effective uniform membranesurface, autodoping to the lower substrate and to the upper device layermust be prevented or minimized. This is accomplished in one method byepitaxially growing a SiGe layer of less than 4,000 Å (1 Å=10⁻¹⁰m) andless than 25% Ge on either side of the barrier etch stop. The SiGelayers and the barrier etch stop layers are subsequently removed afterformation of the membrane in order to complete device dielectricisolation.

The MDI process for forming a dielectric membrane requires that thedielectric material be deposited in net surface tensile stress and thatthe tensile surface stress level be 2 to 100 times less than thefracture strength of the dielectric. Consideration is also given tomatching the coefficient of thermal expansion of the semiconductormaterial and the various dielectric materials being used in order tominimize the extrinsic net surface stress of the membrane. Thermallyformed silicon dioxide forms as a strongly compressive film and mostdeposited dielectrics currently in use form typically with compressivesurface stress. High temperature silicon dioxide and silicon nitridedielectric deposited films with tensile surface stress levels 100 timesless than their fracture strength have been demonstrated as large areafree standing membranes consistent with the requirements of the MDIprocess.

It is not a requirement of the MDI process that if a semiconductormaterial is used in the fabrication of a circuit membrane, that thesemiconductor material be capable of forming a free standing membrane.The dielectric materials that are optionally used to isolate anysemiconductor devices in the circuit membrane can provide the primarystructural means of the resulting circuit membrane as implied by thegeneral fabrication methods presented below.

The ability to form large durable temperature tolerant low tensilestress films of both semiconductor and dielectric materials ascomponents or layers of a substrate for the fabrication of integratedcircuits and interconnect structures is unique to the MDI process. Thelarge free standing semiconductor and dielectric membrane substrates ofthe MDI process provide unique structural advantages to lower the costand complexity of circuit fabrication and enhance the performance ofcircuit operation.

The MDI process can be broadly described as two methods, depending onwhich method of forming the semiconductor substrate thin film isselected. The sequence of steps of the two process methods presentedbelow may be utilized in a different order depending on processingefficiencies. Formation of polysilicon or a-Si (amorphous silicon)circuit devices on a dielectric membrane without the starting use of asemiconductor substrate is disclosed below, but is not categorized as amethod.

Method #1

-   -   1. Implant, diffuse, or epitaxially grow an etch barrier layer        in over the silicon substrate.    -   2. Optionally grow the desired epitaxial device layers.    -   3. Optionally trench isolate the semiconductor device areas.    -   4. Complete all desired IC processing steps including deposition        of a low stress dielectric membrane.    -   5. Form a dielectric and semiconductor substrate membrane by        selectively etching the back side of the substrate to a barrier        layer or to a controlled substrate residual.    -   6. Complete the IC processing steps on the back side of the        substrate and remaining top side of the substrate, and        optionally trench isolate the semiconductor devices if not done        in step 3.

Method #1 fabricates the desired semiconductor devices of an IC on astandard thickness semiconductor substrate 10 prior to the deposition ofthe low stress dielectric membrane and release of membrane structure 14by etching the back side of the substrate. (FIG. 1 a shows incross-sections the substrate 10 after back side etching.) Thesesemiconductor devices will typically b trench isolated (see below)through or below the active area layer of each semiconductor device.

The semiconductor devices will typically be fabricated from vapor phaseepitaxial depositions. Well known epitaxial fabrication methods such asSEG (Selective Epitaxial Growth), ELO (Epitaxial Lateral Overgrowth),MOCVD (Metal-Organic Chemical Vapor Deposition) or MBE (Molecular BeamEpitaxy) may be employed.

In the embodiment of FIG. 1 a an etch stop barrier layer 12 is implantedjust beneath the surface of the silicon substrate 10. This is shown inFIG. 1 b, which is an expanded view of a portion of substrate 10 priorto the backside etching step. Portion 11 of substrate 10 is to beremoved. This barrier etch stop layer 12 may be an oxide layer producedin a manner similar to the well known SIMOX technique or an implantedsilicon nitride layer; however, the thickness of the buried oxide ornitride. layer required for MDI processing is significantly less thanthat required to achieve device dielectric isolation for a siliconsubstrate which is the general application for such implant technology.(The-term SIMOX (Separation by IMplanted OXygen) is a general referencefor the dielectric isolation process of forming a buried layer of SiO₂in a bulk silicon substrate by oxygen implant.) The purpose of layer 12is to induce minimum damage to the crystalline surface of the substrate10 which would degrade subsequent semiconductor device processing, whileproviding a uniform substrate (silicon) preferential back side etch stopjust beneath the surface of the substrate. The barrier etch stop layer12 can be made at a well controlled thickness and acts as an end pointwhen the silicon substrate 10 is selectively etched from the back side14 (see FIG. 1 a) as part of the processing in forming the low stressdielectric and (optionally) semiconductor circuit membrane. The barrieretch stop 12 as shown in FIG. 1 b does not extend to the edge of thesubstrate. It is not restricted from extending to the edge of thesubstrate (wafer), but it must extend beyond that portion 11 of thesubstrate 10 that will be removed during the formation of the membranestructure.

The original substrate upon which the MDI circuit membrane is formedserves as retaining frame (or ring) 18 for the MDI circuit membraneafter the backside of the substrate is etched. The backside etch of thesubstrate leaves the frame 18 of the substrate to hold the resultingcircuit membrane. The width of frame 18 is sufficient to prevent thesurface forces of the circuit membrane from causing the frame to crack.A width typically of less than 400 mils (1 cm) is sufficient. This isdone by growing thermal oxide and or depositing a film typically ofsilicon nitride (about 5,000 Å to 7,500 Å thick) on the backside 14 ofthe substrate and then etching a window or opening in dielectric layerthe size and shape of the desired circuit membrane. The substrate isthen etched through dielectric window or dielectric mask. The shape ofthe backside window opening (or dielectric mask) is typicallyrectangular, although it can vary in shape.

The barrier etch stop layer 12 is then selectively etched to leave onlythe original surface 10 layer of the silicon substrate. A barrier etchstop layer could also be formed on the surface of the silicon substrateif the layer preserves sufficient crystalline structure required forsubsequent epitaxial processing, surface stress (typically less than 10⁸dynes/cm² tensile) and there is a selective etch procedure for thesilicon substrate versus the barrier etch stop layer, and if need be,the applied barrier etch stop layer can subsequently be removed. Thebackside 14 is etched away by TMAH (tetra-methyl ammonia hydroxide) orother appropriate selective silicon etchant (see below). FIG. 1 c showsformation of the semiconductor devices 24, 26, 28 in the semiconductorsubstrate after backside substrate etch to a buried etch stop layer,dielectric deposition, and selective removal of the barrier etch stop.Trenches 25, 27, 29 are cut into the substrate 10 prior to deposition ofthe low stress dielectric membrane 20, which also serves as an isolationdielectric between adjacent semiconductor devices 24, 26, 28.

An alternative technique to trench isolation as suggested by eithersteps 3 or 6 of Method #1 in order to laterally isolate transistordevices of the circuit membrane is to use the well known LOCOS (LOCalOxidation of Silicon) isolation method. LOCOS could be applied as partof the processing of step 6 (process steps 4 or 5 in Method 12 below) asshown in FIGS. 1 d and 1 e. FIG. 1 d shows in cross-section severaltransistors 11 a, 11 b, 11 c of a circuit membrane 20 with semiconductorlayer 20 b, interconnect metallization 20 c, low stress dielectricmembrane 20 d, and with completed topside device processing and abackside deposited low stress silicon nitride mask 13 patterned withopenings 15 a, 15 b between transistors. Subsequent thermal oxidation 17a, 17 b of the thin silicon device substrate layer underneath theopenings in the nitride mask 13, as shown in FIG. 1 e, laterallyisolates the transistors 11 a, 11 b, 11 c and results in completedielectric isolation of the circuit membrane transistors 11 a, 11 b, 11c.

The thermal oxide isolation created by the LOCOS method may change thenet tensile surface stress of the semiconductor (substrate) membranelayer. The deposition of low stress dielectric films on either side ofthe semiconductor layer prior to LOCOS processing will offset mostcompressive effects of the oxide formation. Device isolation by theLOCOS process when applied to a MDI circuit membrane is more effectivethan current bulk processing because of the shallow depth of thesemiconductor substrate layer. This also allows near optimum density ofthe circuit devices with respect to minimum device isolation separation.The easy incorporation of LOCOS into the MDI processing methods also isan indication of the general compatibility of the MDI process withexisting IC fabrication techniques.

The remaining substrate 10 (see FIG. 1 f) is then (optionally) bonded atits edges to a bonding frame or ring 19 (which is typically of glass,quartz or metal and about 25 to 100 mills thick) by conventional anodic,fusion thermal or epoxy bonding techniques. In this case the substrateframe 18 may be removed (see FIG. 1 g). The bonding frame or ring 19 isnot required for processing MDI; the original substrate 10 (which isbackside etched) performs this function initially.

An alternative to forming a barrier etch stop layer is to polish thesilicon wafer 10 on both sides to a thickness uniformity specificationsuch that when the substrate is selectively time etched from the backside, the etch can be stopped prior to reaching the dielectric membrane20 and will leave an acceptably uniform thickness for the active devicesubstrate. This method is mechanically difficult in that it requires thewafer to be exactingly polished on both sides to achieve the uniformthickness.

The dielectric membrane 20, 36 is formed as part of the interconnectmetallization dielectric and as a layer over the two-sided interconnectmetallization 35 as shown in FIGS. 3 a and 3 b. (FIG. 3 b is an enlargedview of the portion of FIG. 3 a referenced as “3 b”.) The thickness ofthe dielectric membrane 20, 36 may vary from less than 2 μm to over 15μm per layer of interconnect metallization layer 35. All blanketdielectric material covering the substrate has a low stress surfacetension and is preferably in tensile stress.

Two sided interconnect metallization 35, with low stress dielectricmaterial 36 (see FIG. 3 b) used as th interconnect dielectric can beapplied, or the back side of the circuit membrane can be passivated withlow stress dielectric for a conventional one-side interconnectstructure. The circuit membrane can withstand processing temperatures inexcess of 400° C. which is required for further deposition of variouslow stress dielectric layers such as SiO₂ and Si_(x)N_(x) and to achievereliable low resistance junctions between semiconductor device electrodecontacts and interconnect metallization. Depending on the specifiedcomposition and thickness of the dielectric and substrate membranematerial, higher temperature processing steps can be performed such asmay be required by implant activation annealing or epitaxial processing.

The alignment of lithography tools on the back side of the substrate isperformed through the transparent dielectric material as shown in FIG.4. The semiconductor substrate material 10 a near the alignment mark 40(which is nearly transparent in the visible portion of the spectrum atthicknesses less than 2 μm) can be etched away from the dielectricmembrane 20 so that the lithography alignment marks 40 are more readilyvisible. Infrared alignment mechanisms are also available for back sidealignment; silicon is transparent at infrared wavelengths. Lithographymarks 40 for infrared alignment cannot be as small as those used in thevisible spectrum due to the longer wavelengths of the infrared,typically greater than 6,000 Å.

Method #2

-   -   1. Form a free standing low stress semiconductor substrate        membrane.    -   2. Optionally grow the desired epitaxial device layers.    -   3. Complete all desired top side IC processing steps including        deposition of a low stress dielectric membrane.    -   4. Optionally trench isolate the semiconductor device areas from        the back side.    -   5. Complete IC processing steps on the back side of substrate        and remaining top side of substrate.

IC processing steps used on the top side and backside of thesemiconductor membrane substrate are well known and not unique inapplication to the semiconductor substrate membrane; nearly anysemiconductor process technique can be applied.

Method #2 fabricates a thin low stress semiconductor substrate membrane20, such as the one shown in FIG. 2, prior to deposition of the lowstress dielectric membrane and fabrication of semiconductor devices.This substrate membrane as shown in FIG. 2 can be formed throughestablished selective electro-chemical etching techniques or bycombination of wafer bonding, grinding and selective substrate etchingtechniques. The membrane preferably has a tensile stress ofapproximately 10⁸ dynes/cm². (This can be achieved in one manner byforming a layer with n-type dopant concentrations of 10¹⁶ to 10¹⁹atoms/cm² and applying electrochemical etch techniques.) After thesemiconductor substrate membrane 20 is fabricated (see FIG. 3 a),semiconductor devices 24, 26, 28, . . . , 30 are fabricated andinterconnected on the substrate 20 with the use of lo stress dielectricmaterial. The low stress dielectric membrane formed on the semiconductorsubstrate (along with interconnect metallization) becomes the onlystructural circuit membrane component after the semiconductor substrateportion of the membrane is etched or trenched into independentsemiconductor devices. An additional layer (not shown) of low stressdielectric over the interconnect metallization may be applied forpassivation and to increase the thickness of the resulting membrane 20to achieve a specific desired level of durability.

The back side etch of the semiconductor substrate 14 (see FIG. 2)permits the under side of the semiconductor devices 24, . . . , 30 to beaccessed and additional processing of the semiconductor devicesperformed. The processing options on the under side of the semiconductordevices 24, . . . , 30 are the same as those for Method #1; however,trench isolation (see FIG. 1 c) of the semiconductor devices has not asyet been performed. Trench isolation is the etching of a separationspace 25, 27, 29 (typically less than 2 μm wide) between semiconductordevices 24, 26, 28 on, all sides of the semiconductor devices and is anestablished IC process technique. The space or trench 25, 27, 29 is thenfilled with dielectric. Trench isolation is novel as applied here,because the technique is being applied to a semiconductor membranesupported by a dielectric membrane. If trench isolation is desired,established masking and etching techniques can be applied to formtrenches.

The uniformity of the semiconductor substrate thickness is important tothe uniformity of the operating characteristics of the semiconductordevices of an IC, and to lithography processing steps. Theabove-described embodiments use etch stop techniques when selectivelyetching the semiconductor substrate material to form the desiredsubstrate membrane thickness.

The interconnect circuit membrane can also be formed on a glass orquartz substrate 23 (typically less than 50 mils thick). In this case atensile film 25 of polysilicon (as taught by L. E. Trimble and G. E.Celler in “Evaluation of polycrystalline silicon membranes on fusedsilica” J. Vac. Sci. Technology B7(6), November/December 1989) isdeposited on both sides of the substrate 23, as shown in FIG. 1 h. A MDIinterconnect circuit membrane 27 is then formed on the polysiliconmembrane 25, as shown in FIG. 1 i. An opening 29 in the polysilicon 25on the backsid of the substrate 23 is made and the back of the substrate23 is selectively etched leaving a free standing tensile polysiliconmembrane 27 held in the frame 23 b of the remaining substrate, as shownin FIG. 1 j. The polysilicon 25 directly beneath the circuit membrane 27can optionally be removed as also shown in FIG. 1 j. A metal film (notshown) or other protective layer is deposited over the circuit membraneprior to etching the backside of the substrate to protect the circuitmembrane from the substrate etchant. Considerations for using anon-semiconductor substrate in the fabrication of MDI interconnectcircuit membrane are substrate material cost or applicationrequirements. Examples of applications for this approach to membranefabrication are masks for high resolution conformal contact lithographicprinting and MCM (Multi-Chip Module) interconnect circuits.

Low Stress Silicon Dioxide and Silicon Nitride Deposition Recipes

Low stress dielectric membranes have been manufactured consistent withMDI requirements for low stress high temperature dielectric films. Thesemembranes were produced on Novellus Systems, Inc. (San Jose, Calif.)Concept One dielectric deposition equipment, but are not limited to suchequipment. Low stress is defined relative to the silicon dioxide andsilicon nitride deposition made with the Novellus equipment as beingless than 8×10⁸ dynes/cm² (preferably 1×10⁷ dynes/cm²) in tension.Acceptable surface stress levels of different dielectrics made onvarious equipment may vary widely.

The following are two typical recipes used to produce the dielectricmembranes of silicon dioxide or silicon nitride required by the MDIprocess. Variations of these recipes can be used to emphasize specificcharacteristics of the deposited dielectric membrane; these recipes arenot the only recipes to achieve the requirements of the MDI process onthe Novellus equipment and are not limitations on the MDI process. Smallvariations in the parameters of the recipes can produce changes in thematerial structure, etch rate, refractive index, surface stress, orother characteristics of the deposited dielectric material.

Recipe #1 Recipe #2 Silicon Dioxide Silicon Nitride Temperature 400° C.400° C. pressure 1.8 Torr 2.3 Torr HF RF Power 640 watts 220 watts LF RFPower, 160 watts 180 watts 100 ohms SiH₄ 260 sccm 0.23 slm NH₃ — 2.00slm N₂ 1150 sccm 0.60 slm N₂O 6000 sccm —Structurally Enhanced Low Stress Dielectric Circuit Membranes

FIG. 5 shows a structurally enhanced MDI circuit membrane structure.Structural enhancement of the MDI circuit membrane may prove necessaryfor various applications where stress is applied to the membrane as partof normal operation, such as in pressure sensing or in making contacttest measurements as in IC wafer sort testing. A portion 44 of thedielectric membrane is deposited over the SD layer 24 a with a thicknessgreater than 1 μm, and typically 10 to 25 μm. This thicker depositedlayer 44 of the dielectric membrane is patterned with a mask and dryetched to achieve a honeycomb-like pattern of recesses 46 a, 46 b, 46 c.The depth of these recesses 46 a, 46 b, 46 c is approximately 75% of thethickness of the low stress dielectric 44 in which they are etched.Optional electrical contact 47 is provided and shown as an example of acircuit electrode. The dimensions of the opening of the recesses 46 a,46 b, 46 c are typically two or three times the depth dimension. Adeposition of 1000 Å of low stress CVD silicon nitride may optionally beapplied to form a passivating seal.

MDI Air Tunnel Interconnect Structure

The impedance of metal conductors (traces) that make up the interconnectmetallization between semiconductor devices and passive circuit elementssuch as capacitors or resistors must be given careful designconsideration at operational frequencies in excess of 100 MHz. Thedielectric constant of a dielectric (insulating) material is a primarydetermining factor when consideration is given to the use of thematerial. Polyimide materials conventionally used in the construction ofinterconnect structures have dielectric constants typically rangingbetween 2 and 3.5. The dielectric constants of CVD silicon dioxide andsilicon nitride typically range upward from 3.5. The ideal dielectric isvacuum, gaseous or air with a dielectric constant of approximatelyunity. The fabrication of an interconnect structure that isolates themajority of a conductor's surface area with vacuum or gaseous dielectricresults in a near optimum condition for high speed operation of thatstructure. Trace or conductor structures called “Air Bridges” arefabricated on the surface of an IC contacting the surface of the IC onlyperiodically. This periodic contact provides mechanical support and orelectrical contact with a low net dielectric constant of isolation. AirBridges are conventionally used in the fabrication of microwavecircuits; such circuits have operational frequencies in the GHz range.

FIGS. 6 a, 6 b and 6 c show cross-sectional views of a conductorstructure 50 internal to dielectric layer 52, 52 a, but which has agaseous primary surface dielectric contact with only periodic mechanicalcontact 50-a, 50-b, 50-c, like the conventional Air Bridge. Theconductor 50 is held suspended in the gaseous dielectric withoutcontacting the surrounding or enclosing solid material structures. FIG.6 d is a top view of a portion of a conductor or trace showing columnsupports (and/or via contacts) 50-a, 50-b, 50-c and dielectric supportcolumns 52-a, 52-b. This interconnect structure is herein called an “AirTunnel”, and its method of fabrication is a direct extension of themethods used in the above-described fabrication of the MDI circuitmembranes (dielectric and semiconductor membranes). Also shown in FIGS.6 a, 6 b and 6 c are dielectric membrane 56, semiconductor membrane 58,dielectric support column 52 a, ground plane metallization 60, cavity 54formed by etch removal of a-Si (amorphous silicon), and opening 62 foretch removal of all the a-Si. FIG. 6 b is an end-on view of an alternatstructure to FIG. 6 a. FIG. 6 b shows dielectric plug contact 68 tosupport (suspend) conductor 50. FIG. 6 c shows an extension of thestructure of FIG. 6 a with a second layer of dielectric 70 and traces72.

The Air Tunnel structure can be fabricated on any semiconductorsubstrate in addition to an MDI circuit membrane. The structuraldielectric materials used are selected from the same group of low stressdielectrics used in the fabrication of the MDI circuit membranesdiscussed above. The fabrication method for the Air Tunnel provided herecan also be extended to the gaseous dielectric isolation ofsemiconductor devices and passive circuit elements in a circuitmembrane.

The Air Tunnel structure in one embodiment is fabricated with CVDprocessing techniques; alternatively, ECR (Electron-Cyclotron-Resonance)plasma CVD processing may soon provide an alternative deposition method.The gaseous dielectric separation of a conductor or a semiconductordevice is accomplished by forming a sacrificial CVD film of a-Si,polysilicon or alternate material (typically dielectric material) thatcan be deposited by CVD means and selectively etched versus the otherMDI circuit membrane material layers. Anticipating the use of Air Tunnelinterconnect structures in the fabrication of a circuit membrane,semiconductor devices are isolated by trench isolation of each device,and depositing a thin layer of oxide or nitride (typically less than2,000 Å thick) over exposed device surfaces, and then depositing a filmof a-Si. The thickness of the a-Si film and the width of the isolationtrench are chosen such that the trench is plugged or filled evenlyleaving the surface over the plug relatively planar. This plug techniqueis facilitated by CVD process technology which deposits filmsconformally on all interacting surfaces. Subsequent Air Tunnelinterconnect structures are completed and the a-Si layers are removed bya silicon selective etchant; the etchant accesses the aSi through theetch-vias as explained below.

The fabrication method of Air Tunnel interconnection is independent ofthe underlying substrate. The underlying substrate could b a MDI circuitmembrane as shown in FIG. 6 d (fabrication of which is disclosed above),a conventional IC substrate of semiconductor devices, a MCM circuitsubstrate, MDI tester surface membrane, etc.

The Air Tunnel fabrication process begins. (see FIG. 6 f) with thedeposition of an a-Si film 76 onto a substrate 56 with electrodes to beinterconnected. The a-Si film 76 is patterned with trenches and contactvias. A CVD processed metal film 50, typically tungsten (W), isdeposited over the a-Si. The dimensions of the a-Si trenches and vias inthe a-Si layer are consistent with forming planar plugs. The trenches inthe a-Si 76, once plugged with the metal film 50, become the supportcolumns 50-a, 50-b, 50-c of the conductor, see FIGS. 6 a and 6 d. Theconductors deposited in the vias provide mechanical (column) support andelectrical contact. The metal film 50 is patterned and a second CVDprocessed a-Si film 78 deposited over the metal film 50; a thin layer(not shown in FIG. 6 a) of highly conductive metal like Au or Cu may besputter deposited over the CVD metal (prior to the patterning of themetal film 50) to enhance conductivity of the conductor. The a-Si film78 is patterned as shown in FIG. 6 f, removing the a-Si film 78 fromover the metal conductor 50 and forming a trench 80 along the edge ofthe conductor 50. This trench is plugged by a 3rd CVD layer 82 of a-Si,as shown in FIG. 6 g. The patterning of the second a-Si layer 78 overthe metal conductor 50 does not require a critical alignment; analignment tolerance of +50% of the thickness of the second a-Si layer 78is sufficient.

The resulting three films. 76, 78, 82 of a-Si are relatively planar withminor surface features along the edges where conductors 50 are directlybeneath. These surface features can be reduced by shallow thermaloxidation of the a-Si surface and subsequent stripping of the oxide (notshown).

An alternative method that can be used to planarize the a-Si and metalfilms isto spin coat a thick polymer over substrate 75, low stressdielectric 77, first a-Si layer 79, and second a-Si layer 81 after theapplication of the second a-Si film 81. (This a-Si film 81 is depositedin thickness equal to the combination of the former second and thirda-Si films.) The polymer 85 used is selected to have a RIE rate nearlyidentical to the that of the a-Si film. The polymer 85 is completelyremoved and the etch of the a-Si is continued as desired, see FIGS. 6 hand 6 i.

The a-Si film stack is then patterned to form trenches that will becomethe supporting columns 50-a, 50-b, 50-c of an over layer of low stressdielectric. These support columns are typically placed in anon-continuous manner along conductors 50 (as shown in FIG. 6 d), andperiodically placed across open areas 51 where there are not conductors.The periodic and overlapping placement of the support columns(etch-vias.) increase the ability of a-Si selective etchant to removeall deposited a-Si material. The thin metal film 60 (see FIG. 6 a) oftypically less than 5,000 Å thickness may optionally be deposited overthe patterned a-Si prior to the deposition of the low stress dielectricfilm to act as a ground plane and EM (Electro-Magnetic) shield for theconductor 50. (This metal film remains in contact with the overlying lowstress dielectric film after the a-Si has been removed.) A low stressdielectric film 52, 52 a is deposited of typically 1-2 μm thickness overthe patterned a-Si films. This low stress dielectric layer 52, 52 a ispatterned with trench openings 62 to the a-Si films. These trenchopenings are called etch-vias and provide access for a a-Si selectiveetchant like TMAH or ethylene diamine to remove all of the internallayers of a-Si films from each layer of interconnect without regard tothe number of layers.

The etch-vias 62 in the dielectric are placed near dielectric supportcolumns as shown in FIGS. 6 c and 6 d. The etch-vias 62 are placed onevery interconnect layer and with sufficient frequency per layer toallow etchant to reach all lower layers of the Air Tunnel interconnectstructure. After the etch-vias 62 are formed an additional interconnectlayer 72, 80 (see FIG. 6 c) can be formed by repeating the sequence ofprocess steps as outlined below. When the Air Tunnel interconnect.structure is used on a semiconductor membrane to form a circuit membrane(MDI process), it should be clear that the application of the Air Tunnelto both sides of the semiconductor membrane is a natural extension andnot a special case application requiring a different processingsequence.

The etch-vias 62 serve as access ports for gaseous material (such as N₂or air) which allow the effective circuit dielectrics to enter thecircuit membrane. The etch-vias 62 also serve to maintain equalizedpressure between the internal interconnect structure and the externalenvironment of the surface of the circuit.

The following sequence of processing steps for fabricating an Air Tunnelassumes that the substrate being processed has an existing layer of lowstress dielectric film on which to start:

-   -   1. Pattern dielectric for metal contact plug vias and etch-vias.    -   2. Deposit a film of a-Si; the thickness of this film will        determine the lower separation distance of the tunnel conductor        from the underlying low stress dielectric.    -   3. Pattern a-Si film for conductor support columns (vias to        underlying dielectric).    -   4. Deposit metal conductor film.    -   5a. Pattern metal conductor film.    -   5b. Optionally deposit high conductivity metal film before        patterning conductive film.    -   6a. Deposit a-Si film approximately the thickness of the metal        film.    -   7a. Pattern a-Si film to expose conductor surfaces and trench        etch along sides conductors.    -   8a. Deposit a-Si film to planarize the surface and separate the        conductor from the overlying dielectric film.    -   9. Pattern a-Si film for dielectric support columns (vias to        dielectric).    -   10. Optionally deposit ground/shield metal film.    -   11. Deposit low stress dielectric film.    -   12. Pattern dielectric with etch-vias or repeat from first step        above.

Steps 6 through 8 from above can be replaced with the followingalternative steps for forming a planarized conductor layer:

6b. Deposit a-Si film with a thickness equal to the thickness of theconductor and the desired separation over the conductor.

7b. Deposit planarizing polymer with a etch rate that is very similar tothat of the a-Si film, as shown by FIG. 6 h.

8b. RIE until all polymer material has been removed, as shown by FIG. 6i.

The principle attributes of the Air Tunnel are:

-   -   1. Low average dielectric constant (approaching unity).    -   2. Self planarization fabrication method.    -   3. Integral ground or EM Shielding plane.    -   4. Application to one or both sides of a circuit membrane.    -   5. Application to MDI circuit membranes or standard wafer        substrates.

A MOSFET device connected by Air Tunnels is shown in cross-section as anexample in FIG. 6 e. The transistor of FIG. 6 e is formed from asemiconductor membrane and held suspended by the metal contacts to itselectrodes with a gaseous dielectric separation on all remaining devicesurfaces. Shown in FIG. 6 e are the active portion 84 (MOSFET) and theunused portion 86 of the semiconductor membrane, a thin nitrideisolation film 88, mechanical conductor support 94, conductor trace 92,and additional air tunnel interconnect structures 92, 94, 96.

MDI Circuit Membrane Advantaaes

The fabrication of circuit membranes provides the capability tofabricate and use integrated circuits in novel ways such as:

1. Back side interconnect metallization, backside SD electrode contact,and through-the-dielectric membrane interconnect metallization signalrouting. (See FIG. 3 b).

2. The use of optical communication (see FIG. 7) in a vertically arrayedstack of MDI circuit membranes 150 a, 150 b, 150 c with optical transmitand receive semiconductor devices on either side of the circuitmembranes. Busing of data through a stack of several MDI circuitmembranes 150 a, 150 b, 150 c from any point on the surface of an ICrather than forming connections at the edge of ICs (which is currentlythe practice) simplifies the structure of the circuit. An externalcommunication optical transceiver 152 is provided. MDI circuit membrane150 b includes a transparent window 154 through which opticaltransmissions from transmitters (laser diode arrays) 156 pass to opticalreceiver SDs 158. The stack of MDI circuit membranes is held together bysupport 160. The ability to use optical means rather than metalconductors to transmit information between ICs shortens the length ofthe communication path, increases the speed and bandwidth of thecommunication path, and lowers the power consumed compared to presentmetal connection methods.

The optical receiver of the MDI circuit membrane is typically less than5,000 Å in thickness. A receiver of this thickness will absorb only aportion of the optical signal fluence striking it, the remainder of thefluence will pass through and out the opposite side of the receiver.This permits a second or third receiver to be positioned in the path ofthe optical transmitter to receive the same signal. The limitingconditions that determine the number of receivers that can be associatedwith one transmitter are the output fluence of the optical transmitterand the thickness of the transceiver substrate. This ability tooptically transmit to several receivers simultaneously has the benefitsof reducing circuit complexity (versus a transceiver structure at everycircuit membrane interface) and performance (no transceiver (repeater)propagation delay).

3. The vertical bonding of two or more circuit membranes to form a threedimensional circuit structure, as shown in FIG. 8. Interconnectionbetween the circuit membranes 160 a, 160 b, 160 c including SDs 162,164, 166 is by compression bonding of circuit membrane surfaceelectrodes 168 a, 168 b, 168 c, 168 d (pads). Bonding 170 between MDIcircuit membranes is achieved by aligning bond pads 168 c, 168 d(typically between 4 μm and 25 μm in diameter) on the surface of two MDIcircuit membranes 160 b, 160 c and using a mechanical or gas pressuresource to press the bond pads 168 c, 168 d together. If the pads 168 c,168 d are solder, they.can be heated to the melting point of the solder(typically less than 350° C.) causing the pads to weld to each other. Ifthe pads are indium, tin, or alloys of such metals, a bond 170 will formbetween the metal pads 168 c, 168 d with the application ofapproximately 100 p.s.i. pressure and an application temperature between50° C. and 400° C., depending on the metal or alloy selected.

4. MDI Nanometer width gate MOSFET device fabrication methods. Theability to fabricate a MOSFET device with a gate region length of lessthan 0.5 μm (500 nm) presently requires lithograph tools with greaterresolution capability than the current high volume optical steppertools. Several methods are presented here for the fabrication of MOSFETwith gate lengths of less than 500 nm and capable of gate lengths lessthan 25 nm. These methods take advantage of the MDI process to formtransistor gate regions of less than 500 nm (0.5 μm) withoutlithographic means are described hereinafter.

5. Sensor diaphragms formed with integrated circuits fabricated onconventional rigid substrates.

6. Fabrication of electrically isolated semiconductor devices on bothsides of the semiconductor layer of a circuit membrane.

V-Groove Transistor Region Gate Formation Method

FIGS. 9 a to 9 e show a sequence of steps for forming a p-channel orn-channel (npn) transistor with an opposed gate electrode and a gatewidth of less than 25 nm (250 Å). The process steps (see FIG. 9 a)assume a starting substrate of an MDI circuit membrane withapproximately 0.5 to 1.5 μm wide isolated metal gate electrode 174fabricated on the backside (opposed) of the transistor. This MDI circuitmembrane consists of a lightly doped <100> crystalline silicon membranelayer 176 of thickness typically less than 2 μm and a 1 to 2 μm layer oflow stress dielectric 178.

The sequence of process steps for one embodiment is:

-   -   1. Form lightly doped p and n wells. FIG. 9 b shows one such        well 180. This is done by depositing a nitride layer 182 over        the silicon layer 176 of the MDI circuit membrane, patterning        the well 180 and implanting the desired dopant.    -   2. Deposit dielectric 184 (see FIG. 9 c) and pattern an opening        0.75 to 1.25 μm wide over the opposed gate area, but a known        width.    -   3. Anisotropically etch along the <111> crystalline planes of        the silicon to form a V-groove 186 of known depth and side angle        of 54.7 degrees. This is done with a time rated etch.    -   4. Deposit by CVD equipment a metal layer 190 such as tungsten        of 0.5 to 1.5 μm, but of known thickness, just closing the        V-groove 186.    -   5. Etch back the metal layer 190 until a known thickness of        metal plug remains in the V-groove 186. The width of the        resulting transistor gate length is determined by the dimension        of the remaining metal in the V-groove.    -   6. Strip the dielectric mask 184 in FIG. 9 c.    -   7. Etch the silicon 180 surface to just expose the metal plug        190 opposed to the gate 174.    -   8. Deposit a dielectric layer and pattern for heavy implant        doping 192 of transistor source and drain regions. This is done        with n and p doping separately leaving a lightly doped gate        channel region 194.    -   9. Strip the dielectric mask in FIG. 9 e and etch the silicon        180 surface until it is below the level of the metal plug 190,        causing its removal. (This step is optional, the metal plug can        be selectively etch removed.)    -   10. Form trench 196 to isolate transistors to under the layer of        low stress dielectric 178.    -   11. Anneal the active transistor implants 192.    -   12. Form source and drain contacts 198.    -   13. Anneal the source and drain contacts 198.

It should be clear that the. opposed gate electrode which is much widerthat the actual length of the gate width could have been fabricated ontop of the transistor with the source and drain contacts, or after thegate region of the transistor was formed.

The reliable fabrication of the gate region 194 at a known length isdependent on etch rate control. Present etch rate techniques support thecapability to form a gate region 194 length of less than 25 nm. A CVDdielectric could also be used, as the gate implant mask instead of ametal plug 90. This gate fabrication method can also be done in bulksilicon and is not limited to the MDI process. It is important to notethat the sub-micron resolution of the gate region was achieved withoutlithographic means capable of sub-micron resolution.

FIGS. 9 f, g, h, i & j show the process sequence for substrate-imagednonlithographic MOS transistor gate region or Bipolar emitter regionformation. This process takes advantage of the novel ability of the MDIstructure to accommodate process steps on either side of the membranesubstrate. This process can form a minimum feature size of less than 100nm and does so by the use of anisotropic etching epitaxial processes.

FIG. 9 f shows a MDI substrate of semiconductor 189 and low stressdielectric 191 formed by one of the methods presented above. Thedielectric layer 191 on one side has been patterned and thesemiconductor layer 189 anisotropically etched through to the dielectriclayer 193 on the opposite side. The thickness of the semiconductor layer189 is selected to be less than 2 μm, and the dielectric layers 191, 193are less than 1 μm thick each, though these dimensions can be scaled toachieve any transistor dimensions. The size of the patterned opening 195in the dielectric is chosen such that the etched opening 197 onto theunderlying surface of the dielectric is a desired width, less than 100nm. Widths of any dimension can be created, however, widths of less than100 nm are the primary objective of this process which cannot beachieved by optical means. The patterned dielectric 191 has a1,000-2,000 Å layer 199 of low stress silicon nitride to provide highselectivity versus the opposed dielectric layer-during a subsequent RIEprocess step.

FIG. 9 g shows an opening 197 formed through the dielectric layer 193opposite the patterned layer 191. The anisotropically etchedsemiconductor layer 189 with its opening 197 onto the under side of theopposite sid dielectric was used as a substrate-imaging mask, and thdielectric layer 193 was etched by RIE (dry) processing. Thissubstrate-imaged opening 197 can be less than 100 nm in diameter andwill become the gate region of a MOS transistor or the emitter region ofa bipolar transistor. FIG. 9 h shows the substrate-imaged region 197closed by selective epitaxial growth 201; the epitaxial growth 201 canbe formed with a graded dopant structure to lessen short channeleffects. FIG. 9 i shows the formation of a gate oxide 203 and electrode205 after a planarization step of the self-imaged opening 197, CVD orthermal formed gate oxide and formation of gate electrode. It isimportant to note that the dielectric layer 193 in which the gateelectrode 205 is formed is self-aligned to the gate region.

FIG. 9 j shows the finished MOS transistor. The side opposite the gateelectrode 205 is stripped of dielectric, planarized and etched to athickness of less than 2,500 Å. It was then trenched 207 forsource/drain isolation a layer of low stress dielectric 209 depositedand source/drain electrodes 211, 213 formed.

It should be clear to some skilled in the art that a bipolar transistorcan be formed with somewhat similar fabrication steps.

MDI Lateral Epitaxial Grown Transistor Gate Region Fabrication

FIGS. 10 a, b, c, & d show a method for forming a MOSFET transistor withtransistor gate region lengths of sub-micron dimensions, without therequirement for lithography tools with sub-micron CD resolutioncapability. This method is an extension of the above-described MDIprocess technology.

In FIG. 10 a, the starting substrate structure is a combination siliconand dielectric 204 film forming a membrane 202. This membrane 202 isformed by one of the methods disclosed above. Subsequently, membrane 202is patterned for the doping of the n+ and p+ transistor channels. FIG.10 a shows a cross-section of a MOS transistor channel (source 210, gate208 and drain 206 regions) with a nitride mask 212 through which ananisotropically formed gate 208 trench is etched to the underlyingdielectric membrane 204 in the location where the gate region of thetransistor will be constructed. The exposed silicon sidewall 214 may bepolished by a thin thermal oxidation and etch stripping or chemicaltechniques. FIG. 10 b shows a top view of the structure of FIG. 10 a,with the transistor channel 210, 208, 206 isolated from surroundingtransistor devices by dielectrically filled trenches 216. These trenches216 are fabricated prior to etching the gate region trench 208. The gateregion trench 208 is then filled (see FIG. 10 c) from both siliconsidewalls by lateral selective epitaxial growth of silicon 220 and withthe same or similar doping concentration as the adjacent regions 210,206; source and drain implant doping can also be used as a doping meansprior to epitaxially growing the gate region. (In the case of CMOSdevices, the n+ and p+ transistor channels are processed in two separatesequences of processing steps.)

The epitaxial growth process in the gate region trench 208 is stoppedwhen the remaining separation distance “d” between the approachingsilicon sidewalls 220 is equal to the desired gate region length. Thedoping concentration is changed for forming the actual gate region 222,and the epitaxial growth is continued until the gate region trench isclosed, as shown in FIGS. 10 c and 10 d.

The width of the initial gate region trench 208 and the epitaxial growthrate can be controlled with an error of less than 5 nm. This allows thelength d of the gate region to be determined by the precision with whichthe gate region trench 208 can be formed, the deposition rate control ofthe lateral epitaxial growth 220 and the capability to create thedesired doping transitions. The result of this approach means that agate region length d of 25 nm or less can be formed. This is far beyondthe current optical lithographic methods which are presently expected toreach production capability of 120 nm by the end of the decade.

FIG. 10 d shows the completion of the transistor with source and drainelectrodes 226, 224, and an opposed gate electrode 230. The surface ofthe epitaxially grown gate region 222 is not smooth, but the backside ofthe gate region 222 is smooth, due to its formation against theunderlying low stress dielectric membrane 204. The gate electrode 230 ismore easily and reliably formed by patterning the underlying dielectricmembrane 204 from its backside, thereby forming the opposed gateelectrode 230. The backside of the gate region 222 is etched free ofdielectric with an opening significantly larger than the length d of theepitaxially grown gate region 222; portions of the source 210 and drain206 regions are also exposed with the gate region due to lithographictool limitations. Gate oxide isolation 232 is thermally grown ordeposited to the required thickness, and a metal or silicide electrodeis formed 230.

The overlap of the gate electrode 230 over the source 210 and drain 206regions of the transistor channel results undesirably in performancelimiting capacitance. The reduction of the capacitance from theoverlapping gate electrode structure can be achieved by either epitaxialgraded doping of the source and drain regions at the interface to thegate region growth over the gate region, to form an extended gateregion, or by offset of the gate electrode so that the source side edgeof the electrode aligns approximately with the source/gate interface ofthe transistor channel. These methods are disclosed below. Theabove-described MOSFET transistor structure can be converted to abipolar transistor structure as shown in FIG. 12 g, also describedbelow.

The MDI circuit membrane structure provides the capability to positionelectrode contacts of semiconductor devices at their backside (oropposed side). This provides significant savings in reduced processingsteps to achieve device isolation and new transistor structures.

FIG. 11 a shows a MDI circuit membrane (dielectric 240 and semiconductor242 films) in cross-section on which additional semiconductor layerswere formed epitaxially to achieve either an npn or pnp transistorstructure 244. The epitaxial layer 244-a that corresponds to the gateregion can be formed with a thickness of less than 25 nm. FIG. 11 bshows the dielectric 240 and semiconductor 247, 244, 245 membrane aftersubsequent well known semiconductor processing steps which form atransistor channel 244 isolated on the dielectric membrane 240 withsource 246, drain 248 and gate electrodes 250-a, 250-b attached. Alsoshown are source region 245 and drain region 247. The gate oxideisolation 251 is thermally grown or deposited, and gate contacts 250-a,250-b can be formed on one or all sides of the transistor channel.Alternatively, a separate contact 250-b can be formed opposite the gateelectrode 250-a, and used as a transistor bias contact. The opposeddrain contact 248 is formed by etching a via in the dielectric membrane240 underlying the transistor drain region 247.

FIG. 11 c shows a bipolar transistor formed by a well known method buton a low stress dielectric membrane 254, and without the requirement toform a lateral conductive path (typically referred to as a buriedlayer). Formed in semiconductor. film 256 are emitter 258, base 260, andcollector 262 regions. The collector contact 264 is instead formeddirectly under the emitter 258. It should also be apparent that thereare non-silicon semiconductor transistor structures that can berestructured with a backside contact as depicted in FIG. 11 c, such as aGaAs Heterojunction Bipolar Transistor (HBT) or Ballistic Transistor.

The extension of the gate electrode 250-a beyond the gate region in thefabrication of the MOSFET transistor structure of FIG. 11 b increasesthe parasitic capacitance of the transistor, and therefore, adverselyaffect its performance. This capacitance can be reduced by the separateor combined use of several methods referred to as Graded Gate Doping(GGD), Epitaxially Extended Gate (EEG) and Gate Electrode Offset (GEO).The Graded Gate Doping is shown in FIG. 11 b. This method varies thedoping levels of the source 245 and drain regions 247 at theirrespective interfaces to the gate region. This directly decreases theeffective dielectric constant of the semiconductor under the dielectricisolation 251 separating the gate electrode from the source 245 anddrain 247 regions it overlaps.

The Epitaxially Extended Gate 244-b (see. FIG. 11 d) is formed byselective epitaxial growth of the gate region 244-b over the surface ofthe transistor upon which the gate oxide 251 and gate electrode 250-awill be fabricated. This method, more directly than the GGD method ofFIG. 11 b, lowers the capacitance of the semiconductor region directlyunder the gate electrode 250-a where it overlaps the source 245 anddrain 247 regions. The transistor capacitive charging effect contributedby the gate electrode 250-a from source 245 and drain region 247 overlapis decreased by these methods, because of the dopant type and dopantconcentration relative to the polarity of the applied potential on thegate electrode 250-a. The thickness of the epitaxial layer of the EEGmethod is determined based upon the desired operating characteristics ofthe transistor, but typically is less than the dimension of the gateregion length.

The extended gate region 244-b of FIG. 11 d formed by the EpitaxialExtended Gate method can also be a graded doping region. If the extendedgate region 244-b is graded, the dopant concentration decreases goingaway form the gate region (or towards the gate electrode 250-a).

The Gate Electrode Offset (GEO) structure is shown in FIG. 11 f. Shownare source region 269, gate region 271, drain region 273, semiconductorlayer 275, low stress dielectric layer 277, gate oxide 279, and offsetgate electrode 281. The drain side edge of the gate electrode 281 isaligned to the drain/gate 273-271 interface of the transistor channel.The alignment could alternately be mad to the source/gate 269-271interface, and would be a preferred structure under certain devicedesign requirements. The accuracy of the placement of the electrode 281edge is limited by the capability of the lithography tool used. A gapbetween the drain region 273 and gate electrode 281 may result leaving aportion of the gate region 271 not covered by the gate electrode 281 dueto lithography alignment deficiencies. This can be corrected byextending the drain region 273 through implant doping of the exposedgate region. The size of this gap during manufacturing can reliably beexpected to be no greater than the magnitude of the alignmentregistration error of the lithography tool being used.

The transistor structure of FIG. 11 e used as a bipolar transistor is ofinterest in order to achieve well controlled fabrication of thin baseregions. The contact 270 to the base region 272 of a bipolar transistorin FIG. 11 e is achieved by epitaxially extending 272-a the base region.This method like the EEG method is the selective epitaxial growth of anexposed region of the transistor that includes the base region 272.Other elements of FIG. 11 e are collector region 274, collector contact275, dielectric membrane 276, emitter region 277, and emitter contact278.

MDI Transistor Fabrication Based on Confined Laterally Doped Epitaxy

Selective growth of high quality epitaxial films in a confined volumewas reported in a paper (“Confined Lateral Selective Epitaxial Growth ofSilicon for Device Fabrication,” Peter J. Schubert, Gerold W. Neudeck,IEEE Electron Device Letters, Vol. 11, No. 5, May 1990, page 181-183).This “CLSEG” (Confined Lateral Selective Epitaxial Growth) method wasdeveloped to achieve dielectric isolation of crystalline silicon.

FIGS. 12 a, b and c show the use of a MDI circuit membrane to produce, aconfinement cavity for the epitaxial growth of crystalline silicon. Theoriginal objective of CLSEG, to fabricate a dielectric isolatedsemiconductor substrate, is not an objective here because the MDIprocess is itself a dielectric isolation process. What is taught here isthe application of the capability for growing epitaxial films in aconfinement cavity within the context of the MDI process, and aprocessing method for using confined lateral epitaxial growth tofabricate MOSFET transistors with arbitrarily narrow gate region lengthswithout the use of lithographic means.

MDI confined lateral doped epitaxial closely controls transistorgeometry in all three dimensions: height (as deposited semiconductorfilm thickness), width (placement of isolation trenches) and source,gate, drain regions (epitaxial deposition thickness). This makes thedevice operational characteristic more predictable with lowermanufacturing complexity.

The MDI Confined Lateral Epitaxy method begins with a membrane of twofilms, one film 280 of semiconductor (silicon) and one film 282 of lowstress dielectric. This membrane 280, 282 is made by one of the methodsdisclosed above. Dielectric filled isolation trenches 284 (see FIG. 12b, a top view of the structure of FIG. 12 a) are fabricated to form whatwill become the sides of the transistor channel in the semiconductorfilm. Then a low stress dielectric layer 290 typically of 1 μm or lessthickness is deposited, and a window 286 is anisotropically etchedthrough the semiconductor film 280 stopping on the dielectric layer 282and positioned between the isolation trenches. 284, as shown in FIGS. 12a and 12 b. In the preferred embodiment the orientation of the window286 is approximately aligned with the crystallographic orientation ofthe semiconductor film 280 in order to produce a seed crystal walls292-a, 292-b of FIG. 12 c that is at a right angle to the isolationtrench walls 284, 290, 282 in the confinement cavity, as explainedbelow. The transistor depth (thickness) is well controlled by thethickness of the semiconductor membrane 280 which in turn can beprecisely determined by epitaxial and/or etching means.

The exposed semiconductor film 280 is then anisotropically etched backin two directions under the dielectric over layer 290 and along theisolation trenches 284 as shown in FIG. 12 c. This forms a very smoothwalled confinement cavity 292 with a smooth seed crystal 292-a, 292-b atthe end of the cavity 292. The etch is stopped at a lateral depthconsistent with the design requirement for the number of transistors tobe formed in the confinement cavity. The depth “c” of the confinementcavity 292 may vary from about 2 μm to over 15 μm.

A smooth contamination free seed crystal wall 292-a, 292-b is requiredfor the subsequent epitaxial growth. The crystal seed wall 292-a, 292-bwill form at an angle dependent upon the crystallographic orientation ofthe semiconductor film. A <100> silicon film will form a seed wall angleof approximately 54.74° as measured from the horizonal, and a <110>silicon film will form a seed wall 292-a, 292-b with an angle 90° (asshown in FIG. 12 c). GaAs and InP have similar crystallographicproperties and can be used as the.semiconductor film instead of silicon.

The selective epitaxial growth of crystalline semiconductor proceedsfrom the seed crystal wall 292-a, 292-b. The npn or pnp regions ofsilicon MOSFET transistors are epitaxially grown (see FIG. 12 d) with insitu doping to whatever design thicknesses desired. This allows thelength of the base or gate region of the transistor to be determined bythe pitaxial growth process instead of a lithographic process aspresently done. This also allows the length of the base or gate regionof the transistor to be arbitrarily narrow, which cannot be done withcurrent lithography methods. FIG. 12 d shows the confinement cavity 292filled with a CMOS transistor pair of npn and pnp structures 294, 296.Each region of the transistor can be uniquely tailored to length anddoping concentration level, in addition, because the transistor channelis formed by epitaxial means, doping of the transistor 294, 296 regions,can be extended to graded segments as discussed above. Further,non-symmetric doping of source and drain regions can easily beimplemented by time based selection of dopant during epitaxial growth;this is not easily accomplished in bulk wafer processing due lithographylimitations in resolving the gate region.

FIG. 12 d shows two pairs 294, 296 of CMOS transistors formed as mirrorimages of each other in two confinement cavities; this was a designchoice because a single confinement cavity could have been formed byadditional trench dielectric isolation of the initial confinement windowopening. Depending on the method of epitaxial growth, gate regionlengths “g” of less than 25 nm can be fabricated. In contrast, prior artMOSFET transistor gate regions are predominately formed by opticallithographic means and are presently limited to minimum gate regionlengths of approximately 0.5 μm (500 nm).

FIG. 12 e shows the CMOS transistor pairs 294, 296 with electrodecontacts 294-a, 294-b, 294-c, 296-a, 296-b, 296-c. The isolated gateelectrodes 300, 302, 304, 306 each span the narrower gate region lengthof the transistor. The gate electrode is formed by standard lithographicprocesses, and may be many times of greater length than the gate regionlength of the transistor. For example, a lithographically defined gateelectrode of 0.5 μm and a transistor gate region length of 25 nm arepossible. Gate electrode capacitance attributed to the gate electrodeoverlap of the source or drain regions can be reduced as described abovewith the GGD, EEG, and GEO methods. FIG. 12 f shows embodiments of allof these methods; the electrode 312 opposed to the gate electrod 308 ispresented optionally as a substrate bias. Also shown in FIG. 12 f aresource region 294-1, optionally graded doped gate region 294-2,epitaxially extended gate regions 294-4, offset gate electrode 308, gateisolation 310, and optionally epitaxially extended substrate 312-a.

The MDI Confined Lateral Epitaxy method is not limited to MOSFETtransistors or silicon as a semiconductor material. The MDI ConfinedLateral Epitaxy method can be used to fabricate silicon bipolartransistors, or applied to other semiconductors such GaAs or InP whichcan be epitaxially formed. An example of a bipolar transistor is shownin FIG. 12 g when emitter region 314, base region 316, epi-extended baseand electrode 318, collector 315, and optionally epi-extended base andelectrode 319.

The semiconductor layer of a MDI circuit membrane provides variousmethods to pack circuit devices densely, mix semiconductor types orsimplify the fabrication steps. FIGS. 12 h, i & j show examples of suchmethods. FIG. 12 h shows a silicon membrane 289 upon which an epitaxialdielectric 291 and a subsequent layer of epitaxial semiconductor 293 hasbeen formed; the total thickness of this membrane structure is typicallyless than 4 μm. The dielectric layer 291 should closely match thecrystal lattice dimension and structure of the silicon 289; an exampleof such a dielectric with a similar crystal lattice is sapphire orundoped silicon under certain device design parameters. This MDIsemiconductor layer structure allows semiconductor devices to be formedon either side of the semiconductor layer 289 and remain electricallyisolated. An obvious application for this semiconductor or membranestructure is CMOS integrated circuits with p-type device formed on oneside and n-type devices formed on the opposite side. Low stressdielectric would be used to trench isolate individual semiconductordevices and complete the MDI process suggested by Method #2. Further,the second epitaxially grown semiconductor layer 293 can be a materialother than silicon such as GaAs or InP.

FIG. 12 i shows a MDI circuit membrane composed of a silicon layer 295and a low stress dielectric layer 297. The dielectric layer 297 has beenpatterned to form open areas 299 into which semiconductor material wasdeposited by selective epitaxial growth. The selectively depositedsemiconductor 299 can be an semiconductor other than silicon, orsilicon. Semiconductor devices can be fabricated in the epitaxialislands 299 on the backside of the MDI semiconductor layer.

FIG. 12 j shows a MDI circuit membrane composed of a silicon layer 295and a low stress dielectric layer 297. The dielectric layer 297 has beenpatterned and silicon islands 299 formed through selective epitaxialgrowth in the same manner as described for FIG. 12 i. The silicon layer295 was subsequently patterned removing portions of the silicon layeropposite each island 299 that was epitaxially grown in the dielectriclayer, and then filling with low stress dielectric. Circuit devicesformed in the semiconductor islands 299 are dielectrically isolated.This membrane structure has similar applications as those suggested forFIG. 12 h.

The structures FIGS. 12 h, i & j show that a two level device structurecan be formed that permits more compact interconnection of the circuitdevices by the use of vertical interconnections. These structures alsosimplify the fabrication of mixed device technologies such as BiCMOS byallowing the Bipolar and MOSFET devices to be fabricated on either sideof the semiconductor membrane without an increase of interconnectcomplexity. The use of these MDI structures typically doubles the totalsurface area are available for interconnect metallization routing. Theobvious potential processing benefits can be a reduction in the numberinterconnect metallization layers per side and or relaxation of theinterconnect metallization pitch used, both of these benefits normallylead to an increase in circuit yield.

Dielectric Membranes as Sensor on Rigid Substrates

The mechanical (physical) and thermal properties of the nitride andoxide free standing membranes made with the Novellus equipment aresimilar to silicon membranes presently being used in the making of ICswith integrated sensors. Examples of such silicon membrane or diaphragmbased sensors are those made for sensing acceleration, pressure ortemperature, but not limited to these areas. The availability of suchoxide and nitride dielectrics for general application is novel with thisdisclosure.

The use of such free standing dielectrics membranes significantlyreduces the complexity of producing a membrane with non-conductingproperties in a silicon or semiconductor fabrication environment. Thelow stress dielectric membranes have selective etchants that allow thedielectrics to be processed independently in the presents of silicon.The known passivating capability and inert chemical nature of siliconnitride, and the relatively similar coefficient of thermal expansion ofsilicon, silicon dioxide and silicon nitride increases the operatingenvironmental range of sensors versus organic dielectrics.

Sensor diaphragms of low stress dielectric membranes produced on rigidsubstrates (typically semiconductor) can in most cases directly replacesilicon membranes or be used in conjunction with a silicon membraneformed on a rigid (conventional) semiconductor substrate. No newfabrication methods are required to integrate the low stress MDI processdielectrics into such sensor ICs.

Applications of MDI Circuit Membranes

The following discloses additional applications of the MDI process, andof forms of MDI circuit membranes and extensions of the MDI IC process.For example, a MDI circuit membrane with few or without active circuitdevices is called more specifically an interconnect circuit membrane.When the interconnect circuit membrane is used to interconnectconventional ICs in die form, the interconnect circuit membrane becomesa multiple chip module (MCM) interconnect circuit membrane as discussedimmediately below. This does not limit the application of a MDI circuitmembrane with active and passive circuit devices from being used in thefabrication of a MCM.

MDI Multi-Chip Module Interconnect Circuit Membrane

A MDI circuit membrane composed primarily of interconnect metallizationof one or several internal interconnect levels can be fabricated on asubstrate to provide electrical interconnections between the bond(signal) pads of various individual dice (ICs) as in the wellestablished packaging technique called multiple chip module (MCM). A MDIcircuit membrane made for this typ of application is called multi-chipmodule Interconnect Circuit Membrane (ICM).

The multi-chip module interconnect circuit membrane is formed using theMDI process and conventional semiconductor processing techniques. Theinterconnect circuit membrane may have several thousands of die metalbond pad contacts or bonding points. The position of these metal contactor bonding points on the surface of the interconnect circuit membranecan be arbitrarily placed.

The multi-chip module interconnect circuit membrane can be formed on anysubstrate material that can meet the requirements for flatnessspecification, withstand the processing temperatures of the applied lowstress dielectric(s), and can be selectively etched with respect to thelow stress dielectric(s) to form the circuit membrane. FIG. 13 a showsan embodiment of an multi-chip module interconnect circuit membrane madeas described above by the MDI methods. FIG. 13 a includes theinterconnect circuit membrane 320 with several internal metallizationtrace layers (not shown), ICs (dice) 322 a, 322 b, 322 c, interconnectcircuit membrane substrate fram 324, and IC bonding contacts 326. FIG.13 b shows a related structure which is a cross-section of a functionaltester for testing ICs with probe points 330 formed on attached ICs 336a, 336 b.

The interconnect circuit membrane can also be formed (see FIG. 14) usingthe MDI method on a quartz substrate 340 that has been coated with arelease agent 342 such as KBr; this assumes that a single-crystal typemembrane or substrate 340 is not required for the fabrication of varioustypes of active circuit devices (SDs) in the interconnect circuitmembrane 320. Thus this structure is made as described above, butwithout mono-crystalline SDs, as in a MCM that as a minimum requiresonly an interconnect circuit membrane or passive circuit devices andtransistors made from polycrystalline or amorphous semiconductor. FIG.15 shows the multiple chip module interconnect circuit membrane 320 ofFIG. 13 a made with the release agent method of FIG. 14 and held by aseparately formed frame 350 that was bonded to th interconnect circuitmembrane 320 just prior to activating the release agent by scribing atthe shoulder of the quartz substrate, thereby cutting through theinterconnect circuit membrane 320 and applying the appropriate solventfor the release agent. The array of die (ICs) 322 a, 322 b, 322 c canthen be directly bonded (as described below) to the interconnect circuitmembrane 320, or the interconnect circuit membrane 320 can be alignedand mechanically held against the pads of the surfaces of the array ofdie (ICs) 322 a, 322 b, 322 c to achieve electrical contact.Alternately, the circuit membrane made with the method utilizingpolysilicon as disclosed above and shown in FIG. 1 j can also be used asan interconnect circuit membrane to achieve a similar result as thatshown in FIGS. 13 a, 13 b and 15.

The interconnect metallization traces in the circuit membrane formelectrical connections from one or more of the contact pads of an IC toone or more contact pads of the other ICs. The diameter of these padscan vary from less than 0.5 mil (0.001″ or 25 μm) to several mils. Thelimiting factors of the size of the IC contact pads is the lithographymethod used in fabrication and the assembly method; the cost of suchmethods increase as pad diameters less than 3 mils are employed.

A die can be bonded to a circuit membrane with any of severaltechniques. Techniques such as compressive metal to metal bonding withsuch metals as Indium or gold (or their respective alloys), infraredthermal bonding, laser bonding, and vertically conductive adhesive filmssuch as ZAF from 3M Corporation are examples. FIG. 16 a shows thepackaging for a version of the structure shown in FIG. 13 a, includingIC carrier lid 352, compressible material (such as silicone) 354attached thereto, carrier substrate 355 of the MCM package, contacts 356to the carrier signal pins 357. An alternative package is shown in FIG.16 b, with gastight gasket 358 enclosing pressurized volume 359.

If the interconnect circuit membrane 360 (see FIG. 17 a) is made fromoptically transparent dielectrics such as low stress silicon dioxide andor silicon nitride, this permits the conventional die (IC) 362 to bevisually aligned on the interconnect circuit membrane 360 from thebackside or from the side of the interconnect circuit membrane 360opposite that of the die 362. A locally directed infrared or other heatsource (not shown) can then be applied from the backside of theinterconnect circuit membrane 360. On the opposite side (top side) isformed MDI pad 365 which is surrounded by solder wells 364, and solderweld bonds can be formed between the interconnect circuit membrane pads365 and the die bond pads 366 on which is formed solder bump 367 whichtypically is about 5 to 25 μm thick. The die 362 is attached face downagainst the interconnect circuit membrane 360.

Solder weld bonds are formed by using a heat source (not shown) such asa high intensity lamp, laser or metal instrument that can heat thesolder to its melting point. The heat source melts the solder 367 (seeFIG. 17 b) directly under the die pad 360 by heating through theinterconnect circuit membrane 360 while the die is in contact with theinterconnect circuit membrane. The high temperature tolerance of thedielectric material, its innate elasticity and thin structure allow heatfrom the heating source to quickly reach and melt the solder 367 to forma bond of 3 to 10 μm thickness. The melted solder 367 wets both the diepad 366 and the interconnect circuit membrane pad 365, and when it coolsthe solder 367 contracts holding the die pad 366 firmly against theinterconnect circuit membrane pad 365. The solder bond is formed with nomore pressure than what would be exerted when the die 362 is brought incontact with the interconnect circuit membrane 360, pressing into theinterconnect circuit membrane and deflecting it by less than 25 μm. Whenthe solder 367 is melted, the interconnect circuit membrane 360 assumesa uniform planar contact against the surface of the die 362. This alsoremoves the requirement that the solder on each pad be formed to anexacting equivalent height.

The solder 367 (see FIG. 17 c) alternatively is originally electroplatedonto the interconnect circuit membrane bond pad 365. The solder 367 iselectroplated to a height between 5 and 25 μm. The original height ofthe solder bump 367 is greater than the depth of the solder well 364 ofthe interconnect circuit membrane 360 in which the pad 365 of theinterconnect circuit membrane 360 is positioned. FIG. 17 b shows thestructure of FIG. 17 c after soldering or bond formation. If a die isremoved some excess solder 367 will remain on the pad 365 of theinterconnect circuit membrane 360. When a replacement die is bonded tothe same interconnect circuit membrane pad, any excess solder will flowor be pressed into the solder well 364 and not prevent the surface ofthe die 362 and interconnect circuit membrane 360 surface from achievinguniform firm contact with each other. A hermetic seal can be formedbetween the surface of the die 362 and the interconnect circuit membrane360. This is done by forming a metal bond along all edges of the die(not shown). This bond can be soldered, and is formed as describedbelow.

An advantage of the high temperature tolerance of the interconnectcircuit membrane which allows die solder bonding to be done without theapplication of any significant contact pressure between the interconnectcircuit membrane and die, is the ability (see FIG. 18 showing a top viewof bond pads on a die) to place die bond pads 370-1, 370-2, . . . ,370-k, . . . , 370-n directly over the circuit structures of the die(not shown). This allows the placement of semiconductor devices directlyunder a bond pad 370-k without concern that the semiconductor deviceswill be damaged, which is the case when mechanical wire bondingequipment is used to wire bond a die to its carrier. Damage-free bondingis not presently possible with current die wire bonding techniques,because such bonds are compressively formed with a mechanical arm whichpresses, rubbing through the die bond pad into the substrate of the dieto be placed anywhere on the surface of the die. The MDI method makesthe design of the IC easier, removes the need for reserved pad area onthe die (which saves 5 to 10% of the surface area of the die) does notrestrict the placement location of the pad, and allows the pads to belarger since area under the pads is no longer reserved exclusively forthem.

A hermetic seal bond pad 372 extends continuously along the perimeter ofthe surface of the die. A hermetic seal of the surface of the die withthe surface of the circuit membrane can be accomplished by the formationof an enclosing solder bond (comprising a solder well as described abovefor forming pad bonds) along the edg of the die. The hermetic seal istypically also a ground contact for the die.

The infrared solder bonding of die to the multiple chip moduleinterconnect circuit membrane is a simple process which also allows thedies to be easily removed and replaced without damage to theinterconnect circuit membrane. As shown in FIG. 19 a, the IC 372 is heldby a vacuum tool 373 and aligned in close proximity to the MCMinterconnect circuit membrane 374. Once aligned, the interconnectcircuit membrane 374 is gently urged forward by a fluid pressure 376 adistance of several mils and the di 372 is urged toward the interconnectcircuit membrane 374 until contact is established. The infrared heatsource 377 is applied to achiev localized heating of the solder pads(not shown) on the IC die 372 until they melt and wet to theinterconnect circuit membrane 374.

The die 372 can be removed (see FIG. 19 b) by using the same vacuum tool373 to gently pull on the IC 372 while localized heating from heatsource 377 of the IC is applied until the solder bonds melt and the IC372 can be pulled from the interconnect circuit membrane 374; no fluidpressure is applied to the interconnect circuit membrane 374.

The multi-chip module interconnect circuit membrane can be enhanced toinclude passive devices such as resistors, capacitors, and polysiliconor a-Si (amorphous-Silicon) TFTs (Thin Film Transistors). Such circuitelements can be fabricated into or onto the membranes due to the abilityof the dielectric membranes to tolerate processing temperatures inexcess of 400° C. These circuit elements can be fabricated internally tothe interconnect circuit membrane structure as part of various specificlayers or collectively on either external side of the interconnectcircuit membrane. After the interconnect circuit membrane is freed fromthe substrate on which it is initially formed, it is strong enough towithstand continued semiconductor processing steps.

The multi-chip module interconnect circuit membrane can further beenhanced to include any degree of active single crystal semiconductordevices. This is consistent with the earlier discussion of thecapability of bonding two or more MDI membranes to form a threedimensional IC, except in this case conventional dice (ICs) are bondedat the bond pads of the circuitry internal to a MDI circuit membranewhich is also serving the purpose of interconnect circuit membrane.

The multi-chip module interconnect circuit membrane provides novelcircuit cooling advantages due to its low mass structure. The thermalenergy generation of an IC is from the face side of the IC. ICsattached-face down against the circuit membrane allows the thermalenergy of an IC to be radiated and conducted through the circuitmembrane to a liquid or solid heat sink means on the opposite side ofthe circuit membrane. The circuit membrane provides a very short thermalpath from the IC to the heat sinking means. The ability directly to coolICs in this manner is a unique feature of the MDI interconnect circuitmembrane structure. The cooling efficiency that can be achieved by an ICthat is fabricated with the MDI process as a circuit membrane issignificantly improved versus standard thickness substrates by theobvious reduction in the thermal mass resistance of the semiconductorsubstrate.

FIG. 13 d shows in cross-section a portion of a MCM MDI interconnectcircuit membrane 329 including low stress dielectric and interconnectlayer 329 a and semiconductor layer 329 b where two die 331, 333 arebonded face down at their signal pads 335 a, 335 b, 335 c, 335 d. TheMDI circuit membrane 329 incorporates the pad drivers 337 a, 337 b, 337c, 337 d that normally would be incorporated onto the die (IC). Theprimary advantages of integrating the IC pad drivers into the MCM arecircuit performance, simplified use of mixed device technologies(Bipolar & CMOS), cooling and die size (IC real estate).

IC pad drivers are typically designed to meet a broad range of circuitdesign conditions which often results in a trade-off of performance. Thedesigner of the MCM pad driver has a better understanding of theoperational requirements of the pad driver, and therefore, can typicallyoptimize the pad driver design for higher performance. Pad drivers inthe MCM can be formed from Bipolar transistors while the IC can be CMOS.This achieves many of the benefits of BiCMOS without the fabricationcomplexity. The pad drivers of an IC typically generate more than halfof the thermal energy of the IC especially at high operating speeds.Placing the pad drivers in the MCM provides a means to bring the primarythermal components of an IC into direct contact with heat sinking means.IC pad drivers are typically the largest transistor structures making upthe IC. Moving the pad drivers of the IC into the MCM can result in apotential 5-10% reduction in the size of the overall IC.

The incorporation of low complexity ICs into a MCM MDI circuit membranecan reduce overall MCM assembly and parts costs. Such often usedcircuits as bus drivers or combinatorial logic can be incorporated intoa MCM reducing parts and assembly costs without a corresponding increasein the cost to fabricate the MCM. (Once the decision has been made toinclude such circuits in a MCM MDI circuit membrane, there is nosignificant cost difference between the inclusion of 1,000 transistorsor 10,000.) Yield of circuits in.a MCM circuit membrane can be addressedthrough redundant fabrication of the desired circuit devices; thecircuit devices of the MCM circuit membrane comprise only a small amountof the total surface area, typically less than 10%. The circuit devicesof the MCM circuit membrane can be tested for defects. A MDI functionaltester membrane disclosed herein, can be used to perform this testingand, in the same step if so desired, to blow anti-fuses to enable thedefect free circuits or fuses to disable the defective circuits.

The multi-chip module interconnect circuit membrane, like prior art PCBs(printed circuit boards), can provide access to the contact pads of theIC from the opposite side on which the IC is attached. This accesspermits in-circuit testing of the bonded IC to be performed by aseparate interconnect circuit membrane fabricated to do such afunctional test, and also permits electrical continuity testing of thetraces fabricate din the multi-chip module interconnect circuitmembrane, again performed by an interconnect circuit membrane testsurface fabricated for that purpose. FIG. 20 is a top side view of aninterconnect module showing an interconnect circuit membrane surfacebond pad 380 and interconnect circuit membrane feed-through contacts 382from the bond pads of the dice 386-1, 386-2, 386-3 mounted bycompression or solder bonding (see FIG. 21) on the backside of theinterconnect module of FIG. 20. Also shown in FIG. 21 are the backside388 of the etched silicon substrate and the interconnect circuitmembrane surface 190 with the traces 392-1, 392-2, 392-3 interconnectingthe dice 386-1, 386-2, 386-3.

The multi-chip module interconnect circuit membrane method could also beused to construct a functional IC tester surface with a high number ofprobe points (in excess of several thousand) for testing ICs while inwafer or die form (i.e., wafer or die sort testing), and also to wafersort subsections of ICs such as an ALU (Arithmetic Logic Unit), FPU(Floating Point Unit), cache segment, etc.

A functional circuit membrane tester surface made with the MDI processoffers advantages not presently available from present on-wafer ICmembranes testers constructed from polymers such as continuous hightemperature operation (greater than 100° C.) without deleterious effectsdue excessive material expansion or at-speed IC testing without EMinterconnect trace coupling.

High numbers (in excess of 1000) of pads or traces of an IC or ICsubsection can be contacted. The contact sites of the IC (die) can bearbitrary in position and all sites can be contacted at one time. Thecontact sites (pads or traces) can be less than 50 μm (2 mils) or evenless than 2 μm in diameter. The probe points of the functional testersurface can be less than 50 μm or even less than 1 μm in diameter, andcan be placed on center to center distances of less than twice thediameter of the probe points. (Functional probe points can be formed tobe 12 μm in diameter and spaced on center to center distances of lessthan 24 μm.) The ability to form and contact high numbers of pads ortraces with dimensions smaller than presently available in the industryis due to the ability of the MDI IC process to form multiple layers ofinterconnect with established semiconductor fabrication means resultingin a flexible and elastic membrane structure. The multiple layers ofinterconnect provide for denser contact test sites, the thin membranestructure provides for contact conformation with little required force(typically less than 10 psi), and the compatibility of the dielectricmaterial with standard IC fabrication techniques allows contact ofcontact site dimensions less than 2 μm.

A MDI circuit membrane 332 used as an IC functional tester surfaceextends under fluid pressure “P” to make contact with the surface of awafer or single die 339 to be tested as shown in FIG. 13 c. The MDIcircuit membrane 332 can be formed with diameters of approximately thesize of the substrate on which it is formed and to be extended more than40 mils (0.1 cm) depending on the diameter of the membrane. Probe points330 on the tester surface are not restricted in size, and can range insize from greater than 4 mils (0.01 cm) to less than 2 μm in diameter.The number of probe points 330 can vary from less than 100 to severalthousand without restriction and the placement of the probe points canbe arbitrary. These capabilities of the functional tester surface areprimarily due to the structural formulation of the materials used andthe fabrication methods of the MDI process. The MDI functional testersurface can rub through the native aluminum oxide layer of the bondingpads of the die 339 being tested by laterally moving the wafer or die339 holding mechanism several microns (typically less than 10 μm) in aback and forth motion. This is uniquely facilitated by the durability ofthe circuit membrane, the surface flatness of the membrane, theuniformity of probe point height and the low pressure required to extendthe membrane surface so that all probe points are in contact with thesubstrate.

In the case of sub-section testing of a die, the detection of adefective sub-section before packaging permits the option of pinning-outthe IC as one of several alternative product configurations, andthereby, making useable an otherwise scrapped IC.

In FIG. 13 b the multi-chip module interconnect circuit membrane hassignal pad probe points 330 formed on the side opposite of theattachment of dice 336 a, 336 b. The dice 336 a, 336 b of the multiplechip module provide functional test signals to the probe points 330. Thetesting performed by the multiple chip module can be functional testingof a whole die or a subsection of the IC. The probe points 330 arealigned over a die (not shown) to be tested on a wafer, and theinterconnect circuit membrane 332 is extended several mils (nominally15-30 mils) by a fluid pressure as shown in FIG. 13 c. The wafer israised to make contact with the probe points 330, the functional testperformed, the wafer is lowered, positioned to the next IC (die locationon the wafer) and the functional test repeated until all the ICs on thewafer have been tested. Testing die in this manner can also be done ondie that has already been cut from a wafer. This is done by aligning andholding the individual die in contact with the probe points and thefunctional test performed. Piezoelectric horizontal vibration applied tothe interconnect circuit membrane 332 or the wafer or individual diewhile the functional testing is being performed can be used to rubthrough native metal oxide on the die pads.

Alternately the native metal oxide (approximately 30 Å thick) on thepads of a die can be rubbed through by the probe points of thefunctional tester surface by repeatedly moving the wafer or die severalmicrons in a lateral manner when the tester surface is first broughtinto contact with the IC to be tested. The mechanical control presentlyavailable with most wafer or die handling equipment is sufficient toeffect this rubbing action.

The interconnect circuit membrane can range in thickness from less than2 μm to over 25 μm. A typical thickness per layer of interconnect(dielectric and metal traces) is 1 to 4 μm. Dielectric layer of severalmicrons thickness may be used as a planarizing technique to lessen metaltrace step height differentials that can develop when more than twointerconnect layers are used. Planarization of metal traces can also beachieved by depositing the dielectric to a thickness inclusive of themetal trace. A channel that is at least as deep as the desired thicknessof the metal trace is patterned in the dielectric. Then establishedlift-off techniques are used to fill the channel with metal.

FIGS. 22 a to 22 c show this process. In FIG. 22 a, a portion of theinterconnect circuit membrane dielectric 400 is shown with patternedresist layer 402 overlying. Recess 406 is then formed in dielectric 400by isotropic etching to form undercut portions 404 equal nearly to thedepth of etched recess 406. In FIG. 22 b metal layer 408 is deposited.In FIG. 22 c lift off stripping of resist 202 is performed leaving onlymetal trace 408, and the next dielectric layer 410 is deposited.

Planarization of the interconnect structure can be achieved by applyinga thick layer of a planarizing polymer over the MDI dielectric film. Thepolymer must have an etch rate similar to that of the MDI dielectricfilm. The polymer is then removed completely by dry etch means and inthe same process removing any dielectric surface features that extendedinto the polymer layer. This planarization process is identical to theoptional planarization process disclosed in the process steps for theAir Tunnel. The only differences being the material being planarized isa MDI dielectric and not a-Si.

The ability of the interconnect circuit membrane to achieve reliablecontact on the bond pads of a die (see FIG. 13 b) is strongly influencedby the thickness of the interconnect circuit membrane 332 which containsthe probe points 330 for contacting the die to be tested. Aninterconnect circuit membrane 332 may be 25 μm thick or more as requiredfor example by transmission line design considerations of interconnecttraces. Interconnect circuit membrane thickness at the probe points ispreferably less than 8 μm. The structure of FIG. 13 b can be fabricatedas shown in FIGS. 23 a and 23 b.

In FIG. 23 a an etch stop layer 412 (typically metal) is deposited at acertain thickness of the interconnect circuit membrane 332 as theinterconnect circuit membrane 332 is fabricated from repeatedapplications of dielectric and metallization layers to a thickness of 8to over 25 μm. After the fabrications of the interconnect layers of theinterconnect circuit me membrane 332 is completed, an area over theprobe points 330 is masked and etched to the etch stop layer 412 (seeFIG. 23 b). The etch stop layer 412 is then etch removed. Alternatively,the area 414 over the probe points 330 can be etched free of overlyinginterconnect layers as each layer is itself patterned as part of thefabrication of the interconnect metallization. Then dice 336 a, 336 bare attached to the interconnect circuit membrane 332.

MDI Source-Integrated Light Valve (SLV) Direct Write Lithography Tool

The MDI process technology can be used to fabricate a circuit membranewith an nxm cell array of active radiation sources of X-ray, DUV (DeepUltra Violet) or E-Beam sources that can be used to form lithographicpatterns in a film of appropriately sensitive resist. The preferredembodiment in this disclosure uses a X-ray source. Variations of theelectrodes, gas contents and structure of the radiation source cell canbe made within the overall structure of this source integrated lightvalve embodiment. The preference for the X-ray source embodiment is itscapability to pattern a smaller feature size.

The radiation source cells in the membrane are under computer control,so that this type of MDI membrane combines the functions of reticle ormask and a light or radiation source for.fabricating ICs. The radiationsource cells of this MDI circuit membrane are positioned (aligned) overan area of the substrate to be patterned and at a uniform distance fromor in contact with the substrate. Each cell then illuminates (exposes)various portions of the substrate directly beneath it as required. TheMDI circuit membrane functioning in this manner is called herein aSource-Integrated Light Valve (SLV) and can generally be categorized asa Direct Write (maskless) Lithography Tool.

The array of Radiation Source Cells (RSC) that make up the patterningstructure of the SLV, emit a collimated source of radiation. Theexposure feature size of this collimated radiation source isdimensionally much smaller than the area the RSC occupies on the surfaceof the SLV. The emitted collimated radiation source is referred to asthe Radiation Exposure Aperture (REA). The SLV is moved in a scanningX-Y manner over the substrate to be patterned. The dimensions of thescanning motions cause each REA of an RSC to pass over a portion of thesubstrate that typically is equal in area to the size of the RSC. TheRSCs are operated in parallel, and the patterned areas on the substratebeneath each RSC adjoin each other and collectively form a much largeroverall pattern on the substrate. This pattern can be as large as theunderlying substrate. The scanning motion of the RSCs is generated bywell known computer controlled mechanical motor driven stages orpiezoelectric driven stages. The SLV or the substrate can be attached tothe motion stage. Due to the short X-Y motions in the use of the SLV,and the desire to scan the RSCs with REAs of dimensions much less than 1μm and in some cases with dimensions of less than 50 nm, piezoelectricmotion stage drive is the preferred embodiment. As the REAs of the SLVcircuit membrane are scanned over the substrate, they are turned on oroff by computer controlled logic associated with each RSC to effect thedesired exposure pattern to be made on the surface of the substrate. Anexample would be a RSC with an area of 25 μm by 25 μm and a REA of 0.1μm in diameter. In order for the REA to expose an area the size of theRSC, a scanning motion of 25 μm by 0.1 μm would be made 250 times in theX-axis direction while translating 0.1 μm in the Y-axis direction aftereach scan.

FIG. 24 shows a overhead (plan) view of a prototypical SLV 420 with anarray size of 4,096 by 4,096 RSCs, and an overall siz of approximately 5inches by 5 inches. The SLV array 420 of RSCS has associated controllogic 424 for loading the pattern data to each RSC. The SLV 420(including the control logic 424) is a MDI circuit membrane of nominally8 μm to 50 μm in thickness made by established semiconductor processesand held in a rigid frame 426 made from the original substrate on whichthe SLV 420 was fabricated or a frame bonded to the SLV prior to theselective etch removal of the substrate on which the SLV (MDI circuitmembrane) was fabricated. Electromagnetic coupling alignment structures430 (described below) are shown also.

FIG. 25 shows a cross-section of two RSCs 434-1, 434-2 in a SLV 440. ASLV 440 in the preferred embodiment has rows and columns of RSCs 434-1,434-2 where each row and each column can include more than 1,000 RSCsresulting in a total count of RSCs in a SLV in excess of severalmillion. FIG. 25 shows a MDI circuit membrane 440 which is 8 to 50 μmthick, data bus interconnect metallization 442-1, 442-2, control logic424-1, 424-2, X-ray sources 446-1, 446-2, and REAs 434-1 a, 434-2 a.

FIGS. 26 and 27 are cross-sections of two different potentialimplementations of X-ray RSCs 434. FIG. 26 uses a metal cathode 450 witha highly textured surface 452 in the shape of adjacent cubes; eachcorner of the extended portion of each cube is a cold electron pointemitter which emits electrons into the electric field 456 created by thehigh voltage x-ray emitting target 458 beneath it. The size of the RSC434 is approximately 25 μm×25 μm. The thickness (height) of the RSC 434is approximately 8 to 50 μm. The materials used to fabricate the SLV(and RSCS) are those employed to fabricate a MDI circuit membrane asdescribed above—metal 459, low stress dielectric 460 and a singlecrystalline semiconductor membrane substrate 472. The processing stepscan vary widely, but are those used in conventional semiconductor andmicro-machinery fabrication.

The textured cathode 450 will typically be in the shape of a squarearray of rows and columns of metal cubes, with each cube havingdimensions much smaller than the overall dimensions of the squarecathode layer. As an example, if the cathode 450 is 4 μm² in area, thedimension of each cube would be 0.1 to 0.5 μm on a side. The texturedcathode cubes are made by established lithography techniques (E-Beam orDUV optical lithography) and anisotropic RIE processing. This cathode450 configuration is a novel part of the SLV because of high density ofcold electron emitters, and therefore, high electron fluence onto theanode 458. The cathode 450 emits electrons which are accelerated by theelectric field 456 created by the voltage potential of the target oranode 458. When electrons reach the target 458, X-rays are emitted fromthe target 458. X-ray absorber materials 466 such as tungsten or goldare deposited as layers as part of the fabricated structure of the SLVmembrane to provide a means to form the REA 470 and in back scatterlayer 461 to prevent back scattered X-rays from reaching control logicdevices 472 with interconnect 473 to other RSCS in the SLV. This RSC isrelated to the well known X-ray vacuum tube design. The space 474separating the cathode and target is in partial vacuum. This separationdistance can vary from less than 1 μm to 40 μm.

FIG. 27 shows an alternative RSC 434 that uses a laser diode 480 toirradiate onto a target 458 which in turn emits X-rays. The laser diode480 and the target 458 are separated by a distance “d” of 1 or moremicrons in a cavity 482 that is optionally a partial vacuum. Except forthe technique of inducing the emission of X-rays from the target 458material, the RSCs 434 shown in FIGS. 26 and 27 are functionally thesame. RSCs that employ gases such as Cd—Hg or Xe—Hg as in the case ofDUV radiation sources, the structure of the electrodes in the cavity ofthe RSC would change and X-ray absorber layers would not be necessary.RSCs that emit free electrons for resist patterning would require thatthe REA 470 be an opening or that it be closed by a material thin enoughto allow the accelerated beam of electrons to pass through it. If theREA 470 is an opening from the electrode cavity 482 of the RSC, then theSLV would have to be operated in vacuum, just as is the case with E-Beamlithography tools. The Hampshire Corporation (Marlborough, Mass.)presently manufactures an X-ray lithography tool which uses a laserdiode stimulated X-ray radiation source.

Although laser diodes that emit radiation with a wavelength less thanapproximately 6,000 Å do not presently exist, development of shorterwavelength laser diodes is on-going and contemplated as a radiationgenerator for the SLV. There is significant demand for diodes withshorter wavelengths for use in consumer and computer peripheral productslike compact discs. Such devices in the not too distant future may comeinto existence and such laser diodes could possibly be integrated intothe SLV as a radiation source generator of a RSC. The use of laserdiodes is therefore contemplated. An additional application of the SLV(with a RSC of appropriate radiation capability) is radiation-inducedCVD (chemical vapor deposition). The use of an SLV in this mannereliminates the need for mask and etch processing steps. The material ofinterest to be deposited, be it metal or dielectric, is deposited in apatterned form. This provides for a maskless, resistless, etchlessprocess and it has significant cost and particulate contaminationadvantages. The SLV operating in this manner supplies radiation at thesurface of a substrate where a mixture of gaseous compounds are present.This process step is mechanically the same as exposing a resist layer onthe substrate as discussed above. The radiation from a RSC causes thecompounds to react at the surface of the substrate leaving a depositionof material selectively on the irradiated portions of the surface of thesubstrate. This method of radiation-induced selective CVD has beendemonstrated with the use of excimer lasers. Laser diodes that can emitradiation in the excimer frequency band of 75 to 250 nm do not presentlyexist.

MDI Source-External Particle Valve (SPV) Direct Write Lithography Tool

The MDI process technology can be used to fabricate a circuit membranewith an n×m cell array of individually controlled electrostatic orelectromagnetic valves or shutters. These valves can be used to controlthe passage of charged particles through the circuit membrane to createan exposure pattern in a film of appropriately sensitive resist. Thearray of valves on the MDI circuit membrane is for practical purposesthe same structure as the SLV structure shown in FIG. 24. The primarydifference is the use of particle valve cells for patterning an externalparticle source which illuminates one side of the SPV circuit membrane,instead of a cell with an integrated patterning source as in the case ofthe SLV lithography mask.

The array of valve cells of this MDI circuit membrane is placed betweena collimated ion particle source and at a uniform distance from or incontact with a substrate coated with a thin film of resist. The array ispositioned (aligned) over an area of the substrate to be patterned andthe array of valves, under individual computer control, allow ion(charged) particles to expose or not expose an area of resist coatedsubstrate directly under beneath it. The MDI circuit membranefunctioning in this manner is called herein a Source-External ParticleValve (SPV).

The collimated particle beam passed by a particle valve has a fixedexposure feature size called the particle Exposure Aperture (PEA). TheSPV is moved in a scanning X-Y manner over the substrate to bepatterned. The dimensions of the scanning motions cause each PEA of aparticle valve to pass over a portion of the substrate that typically isequal in area to the size of the valve. The valves are operated inparallel (simultaneously), the patterned areas on the substrate beneatheach valve adjoin each other and collectively form a much larger overallpattern on the substrate. The scanning motion of the valves is generatedby well known computer controlled mechanical motor driven stages orpiezoelectric driven stages. The SPV or the substrate can be attached tothe motion stage. Due to the short X-Y motions in the use of the SPV,and the desire to scan the valves with PEAs of dimensions much less than1 mm in diameter and in some cases less than 50 nm, piezoelectric motionstage drive is the preferred embodiment. As the valves of the circuitmembrane are scanned over the substrate, they are turned on or off bycomputer controlled logic associate with each valve to effect thedesired exposure pattern to be mad on the surface of the substrate. Anexample would be a valve with an area of 25 μm by 25 μm and a PEA of 0.1μm in diameter. In order for the valve to expose an area the size of thevalve, a scanning motion of 25 μm by 0.1 μm would be made 250 times inthe X-axis while translating 0.1 μm in the Y-axis after each scan.

FIG. 24 (also a plan view for the SLV) shows an overhead (plan) view ofa prototypical SPV 420 with an array of 4,096 by 4,096 ion or chargedparticle valves, and an overall size of approximately 5 inches by 5inches. The SPV array 420 of valves has associated control logic 424 forloading the pattern data to each valve. The SPV 420 (including controllogic 424) is a MDI circuit membrane of nominally 4 μm to 8 μm inthickness made by established semiconductor processes and held in arigid frame 426 made of the original substrate on which the SPV 420 wasfabricated. Electromagnetic coupling alignment structures 430 (describedbelow) are shown also.

FIG. 29 a shows a cross-section of two valves 502, 504 in a SPV 506. ASPV in the preferred embodiment has rows and columns of valves 502, 504where each row and each column can include more than 1,000 valvesresulting in a total count of valves in a SPV in excess of severalmillion. FIG. 29 a shows a MDI circuit membrane 508 which is 4 μm to 8μm thick, data bus 510 a, 510 b, control logic 512 a, 512 b and PEA(aperture) 514 a, 514 b for passage of ion particles. FIG. 29 b shows aplan view of the valves 502, 504 of FIG. 29 a.

The valves of the SPV prevent the passage of ions or charged particlesby using an electric or magnetic field of the same polarity to deflectapproaching particles from the PEA. The PEA 514 a, 514 b is typically asquare opening in the SPV circuit membrane 508. The PPEA 514 a, 514 b iscircumscribed by a metal film 520 a, 520 b which creates a localelectrostatic field over the PEA by applying a voltage potential, or bya metal wire 540 (coil) (shown in FIG. 29 e) which creates a localelectromagnetic donut shaped field. When the SPV is flooded with acollimated source of charged particles, the particles that fall upon thePEA 514 a, 514 b will pass through the SPV 506 and continue onto theresist layer (not shown) to be patterned directly beneath the SPV 506.If a electrostatic field or electromagnetic field of sufficient strengthand of the same polarity as the ionic particle is present, the ionicparticle will deflect away from the PEA and strike some other portion ofthe SPV surface. The PEA is small in area relative to the totalgenerated electric field volume, therefore, only a small deflection ofthe approaching charged particle is required for it to miss the PEA andstrike the surface of the SPV.

The SPV 506 is typically operated in an enclosure (not shown) undervacuum. The charged ions or particles can be of any material that can beappropriately accelerated or caused to uniformly flood the surface ofthe SPV. Some examples of charged particles that have been used assources for patterning resist films are electrons (negative), protons(positive) and gallium+ (positive). The SPV can be in close proximity orcontact with the substrate to be patterned, or it can be positionedbefore imaging lens elements of the lithography equipment that could beused to enhance the focus of the particles beams formed by passingthrough the SPV circuit membrane or demagnify (reduce) the imagegenerated by the SPV.

FIGS. 29 c and 29 d are cross-sections that show different methods forimplementing an electrostatic shutter or valve of the SPV. FIG. 29 cshows a PEA 514 a formed by an anisotropic etch of the monocrystallinesemiconductor membrane 524; <100> monocrystalline semiconductor wetetches with a wall angle of approximately 54° or dry etching (RIE)methods can also be used to etch a round or square hole to form the PEA514 a in either the low stress dielectric 526 a, 526 b or semiconductorlayer 524 of the SPV circuit membrane. The via in the low stressdielectric membrane 526 b on the backside of the semiconductor membranewas formed after the wet etch of the PEA 514 a. A thin metal film 528 oftypically less than 2,000 Å is deposited by sputtering or CVD means. Themetal film 528 circumscribes the PEA 514 a and has a diameter typicallyof less than 4 μm, and the PEA typically has a diameter d of less than500 nm. FIG. 29 d shows a PEA 530 formed by etching a via in a flatmetal film 532 that was deposited on the semiconductor membrane 534. Themetal film 532 forming the PEA in this example is typically less than2,000 Å thick and has a diameter of approximately 4 μm. Also shown arelow stress dielectric layers 536 a, 536 b.

FIGS. 29 e and 29 f show a method for implementing an electromagneticshutter or valve of the SPV. FIG. 29 e is a plan view of the structureof FIG. 29 f. Shown are semiconductor membrane layer 542 and layer 544low stress dielectric. A metal cbil 540 is formed on MDI circuitmembrane 508 so a current loop can be created around the PEA 514 a. ThePEA 514 a is formed by wet or dry anisotropic etching in either thedielectric or semiconductor portions of the MDI circuit membrane 508.The metal line 540 is typically less than 2 μm wide, less than 1 μmthick and circumscribes a area “a” typically less than 4 μm in diameter.The PEA 514 a is smaller than the inner diameter “a” of the loop of themetal line 540 and typically less than 500 nm.

Control logic is associated with each valve of a SPV, supplying voltageor current as necessary to the metal structure circumscribing the PEAs.In FIGS. 29 c and 29 d the valve control logic applies a voltagepotential to the metal film 528, 532 respectively circumscribing thePEA. If the applied voltage potential is of the same polarity as thecharge particles striking the surface of the SPV, the particles will bedeflected from passing through the PEA. This is the valve closedcondition. If the applied voltage potential is of the opposite polarityor zero, particles will pass through the PEA. In FIGS. 29 e and 29 f thevalve control logic applies a current through the metal wire 540circumscribing the PEA 514 a. Depending on the direction of the currentin the wire 540 (clockwise or counter-clockwise about the PEA) amagnetic field with a polarity is generated. If the field polarity isthe same as the approaching charged particle it will be deflected, andif the field polarity is the opposite or zero particles will passthrough the PEA.

The design of electro-static or electromagnetic valves for achieving thedesired functions of the SPV can vary widely. The examples presentedhere are not intended to limit the scope of the design structure of suchvalves for use in a SPV.

MDI Mechanical Light Valve (MLV) Direct Write Lithography Tool

FIGS. 29 g through 29 k show portions of a direct write lithography toolwhich uses repeated micro-machined mechanical electro-static shuttercell 550-k for the patterning of photonic or particle exposure source.The overall structure and function of this lithography tool is similarto the above described SLV and SPV tools. It is composed of rows andcolumns of pattern forming elements called shutter cells 550-k that areeach approximately 25 μm by 25 μm. FIG. 29 g is a plan view of severalsuch cells 550-1, 550-2, . . . , 550-k, . . . , 550-n. FIG. 24represents a general plan view of the whole lithographic tool, whichlooks the same in plan as the SLV and SPV tools.

An array of shutter cells 550-k made on a MDI circuit membrane is placedbetween a collimated photonic or ion particle source and a substrate tobe patterned, and can be used in projection or contact lithographymethods to pattern such a substrate coated with a thin film of resist.The array is positioned (aligned) over an area of the substrate to bepatterned and the array of shutter cells, each under individual computercontrol regulating the shutter opening or REA (Radiation ExposureAperture) through the circuit membrane, exposes an area of the substratdirectly beneath or corresponding to it. The MDI circuit membranefunctioning in this manner is called herein a Mechanical Light Valv(MLV).

The size of the REA collimated exposure image passed by a shutter cell550-k is variable. Further, the REA does not need to be a physicalopening through the MDI circuit membrane for photonic sources, but onlytransparent to the incident photonic radiation. The MLV is moved in ascanning X-Y manner (shown by X-Y axis 554) over the substrate to bepatterned. The dimensions of the scanning motions cause each REA of ashutter cell to pass over a portion of the substrate that typically isequal in area to the size of the shutter cell 550-k. The shutter cells550-k are operated in parallel (simultaneous) manner; the patternedareas on the substrate beneath each shutter cell adjoin each other andcollectively form a much larger overall pattern on the substrate. Thescanning motion of the shutter cell is generated by well known computercontrolled mechanical motor driven stages or piezoelectric drivenstages. The MLV or the substrate can be attached to the motion stage.Due to the short X-Y emotions in the use of the MLV, and the desire toscan the shutter cells with REAs of dimensions much less than 1 μm indiameter and in some cases less than 50 nm, piezoelectric motion stagedrive is the preferred embodiment.

As the shutter cells of the circuit membrane are scanned over thesubstrate, they are opened or closed by computer controlled logic 556-kassociated with each shutter cell 550-h to effect the desired exposurepattern to be made on the surface of the substrate. An example would bea shutter cell with an area of 25 μm by 25 μm and a REA setting of 0.1μm in diameter. In order for the REA to expose an area the size of theshutter cell, a scanning motion of 25 μm by 0.1 μm would be made 250times in the X-axis while translating 0.1 μm in the Y-axis after eachscan. The size setting of the REA is typically fixed for a given scan ofthe substrate, and vector motion of the REA can be used to improveexposure performance when there are areas of the substrate common to allshutter cells that do not require exposure such as is the case with viapattern layers.

The operation of the shutter cells regulates the opened/closed status ofan opening that allows a fluence of the exposure source (photons orparticles) to pass, and in the same operation dynamically determines theexposure CD for the lithography tool. The ability to change the minimumexposure CD or REA of the MLV is unique to this tool, versus the SLV andthe SPV which have fixed exposure apertures. The ability to dynamicallychange the size of the REA allows the pattern or geometry fracturedatabase of the IC to be partitioned into CD specific geometry sets perpattern layer of an IC. This provides the capability to optimize theperformance of the lithography tool by patterning those portions of anexposure image with the smallest CD (processing rate is due to the sizeof the REA, therefore, the smaller the REA the greater the number of REAexposures), separately from other portions of the exposure imagecomposed of larger CDs.

FIGS. 29 h and 29 i show plan views of two different shutter armstructures of a shutter cell 550-k. FIG. 29 i shows a spring suspensionstructure of the shutter arm 560, 566. The shutter arm 560, 566 isformed of metal or a combination of metal and low stress dielectric andis suspended over the surface of the substrate from two electricalcontacts 562, 564 on either side of the shutter 566. The shutter arm560, 566 can move (in the direction 570-Y indicated) by the switchedapplication of electro-static potentials. The REA 572 of the shuttercell is formed by the intersection of a square opening 574 through thenon-transmissive material of the shutter arm and a similar openingthrough a metal electrode film directly below the central area of theshutter 566 and fixed to the surface of the substrate. This opening 574is typically less than 2 μm in diameter, and is positioned with a 90°rotation relative to the long axis of the shutter arm 560. FIG. 29 hshows in dotted outline 578 as an alternate position of the shutter arm560, 566 produced by the application of electro-static potentials. Thesize of the REA is determined by the projected intersection of theopening in the central area 566 of the shutter and the opening in thefixed metal film beneath it. The REA of the shutter 572 is closed bymoving the central area of the shutter arm 566 a distance greater thanthe diagonal length of the square opening 574.

The shutter arm 560, 566 is positioned or moved by electro-staticpotentials. The shutter arm 560, 566 is provided with a voltagepotential. Electrodes 580 a, 580 b positioned at either end of themaximum travel displacement of the central area of the shutter arm 566are selectively set at opposite potentials sufficient to attract orrepel the shutter arm a desired displacement. The shutter arm is held inphysical contact with the electrode film beneath it by applying anopposite potential to the lower substrate electrode (not shown).

FIG. 29 j is a cross section of part of the shutter cell through linej-j of FIG. 29 h. FIG. 29 k is a cross section along line k-k of FIG. 29j.

As shown in FIG. 29 j, the shutter cell is fabricated with establishedsemiconductor processing methods. The array and shutter cell controllogic are fabricated on a MDI circuit membrane 590. A trench 592 isformed down to a lower dielectric layer 590 of the circuit membranesufficient in size to accommodate the motion of the central area 566 ofthe shutter arm. The electrod 594 under the central area 566 of theshutter arm is formed, and a conformal sacrificial layer (not shown) ofa-Si (amorphous Silicon) is deposited over the area of the shutterarray. The thickness of the a-Si layer determines the separationdistance that the shutter arm is suspended above the circuit membrane.Into the a-Si layer via openings 562, 564 (FIGS. h and i) are patternedto the electrode contacts of the shutter arm 560 and to the electrodecontacts of the shutter arm positioning electrodes 580 a, 580 b. A metallayer 600 optionally combined with a dielectric layer 602 is depositedand patterned to form the shutter arm 560, 566 and the positioningelectrodes 580 a, 580 b. The opening 574 that is used to form the REA isalso patterned and etched via RIE processes. This opening 574 as aminimum passes through the shutter arm and the electrode 594 below thecentral area of the shutter arm that is used in forming thecross-sectioned area that is the REA 572. The opening 574 may becompletely through the MDI circuit membrane 590, or stop on thedielectric layer depending on the transmission requirements of theexposure source. The a-Si is then selectively removed which leaves theshutter arm 560 free standing and connected to the circuit membrane atonly two electrode contact sites 562, 564 (FIGS. h and i). The shutterarm 560, 566 is free to move in two directions 570-Y, 570-Z based on thevoltage potentials applied to the shutter arm 560, 566 positioningelectrodes 580 a, 580 b, and the substrate electrode 594 beneath thecentral portion of the shutter arm.

Fixed Free Standing Membrane Lithography Masks

The MDI process can be used to form free standing membranes used aslithographic masks with fixed patterns. The MDI process can be used toform optical, X-ray and stencil masks lion or charged particle masks).The mask substrate is made from a low stress dielectric and optionallysemiconductor material. In the preferred embodiment masks are made fromoxide and nitride low stress dielectric made on the Novellus equipmentand consistent with the formulation presented above or as variations ofsuch low stress membranes formed on other oxide and nitride fabricationequipment. The patterning material is a semiconductor such as silicon,another dielectric material or a metal which is non-transmissive to aphotonic exposure source. If the exposure source is an ion or chargedparticle beam, then the patterning is accomplished with voids (openingsthrough the membrane) or a lack of material, or stencil.

FIGS. 29 l and 29 m show in cross-section the formation of a fixedphotonic mask. A membrane substrat 620 initially formed of dielectriclayers 622, 624 and silicon 626 (patterning material shown with pattern)is formed as described above. The thickness of the dielectric layers622, 624 of the membrane is typically 2 to 3 μm thick, and the silicon626 may vary in thickness depending upon the desired sidewall angle ofthe silicon or the CD aspect ratio (ratio of layer thickness to the CDopening of the pattern). The silicon layer 626 can be patterned directlywith established resist spin-on films, pattern generation lithographytools such as E-beam and selective silicon etch processing.

FIG. 29 l shows in cross-section a patterned silicon layer 626 formed byfirst patterning an overlay of metal 630 such as tungsten, and thenselectively etching the silicon layer 626 done to the dielectric layer622. A metal pattern could also be formed by removing the silicon layeraltogether and depositing a metal film, then subsequently patterning it,or by selectively undercutting the patterned silicon layer of FIG. 29 lto employ a lift-off process with a noble metal such.as gold.

FIG. 29 m shows in cross-section the structure of FIG. 29 l after thedeposition of a low stress dielectric membrane layer 632 over thepattern and the removal of the dielectric layers 622, 624 below thepattern. The deposition of low stress dielectric membrane layer 632 overthe pattern forms a structural membrane layer so that the original lowerdielectric structural layers 622, 624 could be removed.

In the case of X-ray masks gold is often used as the patterning filmbecause it is a good X-ray absorber. Gold and the noble metals ingeneral can be patterned only with wet processing techniques and or incombination with electroplating techniques. The silicon pattern 626 ofFIG. 29 l can be dry etched (RIE) to achieve significant aspect ratios.If the silicon layer 626 is heavily gold doped prior to RIE processing,this method of mask formation can be used as an X-ray mask without CDfeature size or aspect ratio limitations of wet etch processing.

FIGS. 29 n and 29 p show a stencil mask formed by patterning thedielectric layer 640 of a MDI silicon 642 and dielectric 640 membranewith a RIE process. The silicon layer 640 is then selectively wet etchremoved to leave a stencil pattern as shown by the openings in thedielectric membrane 644 l, 644 b, 644 c in FIG. 29 p. This sequence ofprocessing is novel because it allows the sidewall passivation on thedielectric 640 which forms as a by-product of the RIE processing to beremoved before the fragile stencil pattern is formed. The sidewallpassivation cleaning requires vigorous agitation that would destroy afree standing stencil pattern. The remaining silicon layer 642 can beremoved by gentle silicon selective wet etch.

Stepper Contact Printer (SCP)

Pattern generation tools made from MDI circuit membranes can be used topattern an appropriately sensitive resist film deposited on a substrate.MDI pattern generation tools refer to MDI circuit membrane applicationssuch as the SLV, SPV and MLV lithographic tools, and the moreconventional fixed pattern masks made on a low stress dielectricmembrane with appropriate optical transmission characteristics (seeabove). A MDI fixed pattern mask is a single layer metallization patternformed on a MDI dielectric membrane with one of the MDI fabricationmethods presented above. A method of alignment to an existing image on asubstrate through electrical inductive coupling is presented below; itis also incorporated into the MDI circuit membrane fixed patterngeneration tool.

Contact printing methods are inexpensive and can provide nearlyunlimited lithographic feature size imaging capability. However, the useof thick mask plates for image transfer to the resist on the substratemust come in contact with the resist to achieve the best image results.This contact is brought about by the application of strong vacuum forcesthat bend the rigid mask plate into conformal contact with thesubstrate. The result of applying such strong vacuum forces often causessmall particles of the resist to adhere to the rigid mask which in turnrequires the mask to be cleaned or, if undetected, prevents correctimage transfer by nonconformal contact to the substrate or an imagingdefect during subsequent exposures. Proximity printing is similar tocontact printing except the mask is not brought into contact but is heldseveral micrometers over the substrate called the proximity gap. Theeffectiveness of proximity printing to produce small feature size imagesis directly proportional to the size of the proximity gap. Further,contact printing has been limited to printing a complete substrate inone exposure instead of smaller portions of the substrate as is the casewith conventional stepper projection lithography equipment. The time topull vacuum during the contact printing process is lengthy, andtherefore, making several contact print exposures on a single substratewould be prohibitive in time without consideration to the compoundedmask cleanliness challenge and the mechanical complexity of vacuumapplication to uneven or overlapping ridge mask and substrate plateduring contact.

FIG. 30 shows a cross-section of a stepping contact lithographicexposure equipment that combines the well known contact or proximitylithography methods with a stepper mechanical motion and the patterngeneration tools and alignment means of MDI circuit membranes presentedherein. The substrate (workpiece) 662 is held by a conventional vacuumchuck (not shown) and is positioned by conventional mechanical andpiezoelectric motion control mechanisms (not shown). The MDI patterngeneration tool 660 is held over the substrate 662 a distance “d” ofless than 25 μm and rough alignment over a portion of the substrate 662is made by conventional optical means. The center of the MDI patterngeneration tool 660 is then extended by applying a small fluid pressureP of less than 100 g per square cm to lower the circuit membrane into adesired proximity distance position shown by dotted line 668 orconformal contact with the substrate 662. Final precision alignment ofthe pattern generation tool 660 to the substrate is accomplished by theinductive coupling (electromagnetic) means disclosed below. The low massand elastic nature of the MDI circuit membrane allows the patterngeneration tool to be rapidly brought in and out of contact with thesubstrate. Once an exposure is complete, the pattern generation tool isbrought out of contact with the substrate and stepped (moved) undercomputer control to the next area on the substrate to be exposed. Alsoshown are circuit membrane frame 672 and a lithography equipment fixture676 for holding the pattern generation tool 660, 672.

Electro-Magnetic Lithogrphic Alignment Method

The lithographic pattern generation tools disclosed herein as the SLV,SPV, KLV, and fixed masks are capable of pattern generation withCritical Dimensions (CDs) or minimum pattern feature sizes of less than50 nm. A fixed pattern generation tool, commonly referred to as a mask,can be made with the MDI circuit membrane.process (as disclosed above)with the capability of pattern generation CDs directly proportional tothe wavelength of the exposure source. Potential exposure sources areUV, DUW, E-Beam and particle beam. (Fixed masks made with the MDIprocess are circuit membranes of typically less than 4 μm thickness andwith a single layer of patterned material to be used as in conventionallithographic masks. Alternately, the MDI circuit membrane can be astencil pattern, where the pattern to be generated is represented in thecircuit membrane as trenches that go completely through the membrane.Fixed pattern stencil membranes are required for E-Beam or particle beamlithographic process, where the exposure source must physically passthrough the mask to be imaged (patterned). Prototype particle beamstencil masks have been developed by Ion Microfabrication Systems.)

The utility of the CD generation capability of a pattern generation toolfor the manufacture of ICs, however, is dependent on the ability toalign or register multiple overlaying patterns. Accepted patternalignment tolerances used presently in the semiconductor industry aretypically ±25% of the CD value for a set of overlaying patterns.

Alignment for pattern generation with CDs of approximately 1 μm orgreater can be done with optical microscopes by overlaying or matchingalignment patterns on the substrate with corresponding patterns on thesurface of the pattern generation tool. The microscope can view throughthe pattern generation tool, since it is typically made from transparentdielectric materials.

The preferred embodiments of the SLV, SPV, and MLV are to provide thecapability to generate patterns that have critical dimensions of lessthan 100 nm. Such patterns require alignment methods capable ofachieving pattern registration of less than ±25 nm. Optical alignmentmethods presently available cannot achieve such critical dimensions. TheMDI pattern generation tools achieve alignment for critical dimensionsof less than 100 nm through the use of electromagnetic proximitysensing. FIG. 28 a shows an example of a PGT alignment coil 494. Thismetal coil 494 is fabricated by patterning a thin metal film (typicallyless than 1 μm in thickness) on the surface of the substrate beingpatterned (i.e. on the wafer). The placement of the coil patterns 494 onthe substrate is typically along two adjoining sides of the area to beexposed or patterned. Coil patterns can also be placed within the areaof the substrate to be patterned; they are not restricted to the edge orsides of the pattern area. The coil patterns are typically less than 800μm by 100 μm. The electrode contact pads 496, 498 of the coil area 494are typically 50 μm by 50 μm in area. A similar pattern 494corresponding to structure 430 in FIG. 24 is on the surface of thepattern generation tool membrane (not shown) plus two electrode probepoints that extend down from the surface of the circuit membrane of thepattern generation tool. These pattern generation tool probe points makecontact with the electrode pads associated with the substrate coil 494,and electrical signals are generated in the substrate coil 494. Itshould be apparent, though less convenient, that signals can be suppliedto the substrate by connections from the edge of the substrate and notdirectly from the pattern generation tool.

The signals from the substrate coil are sensed by the correspondingalignment coil in the pattern generation tool. The pattern generationtool or the substrate are then moved in an X-Y and angular manner untilpredefined electrical parameter settings are achieved via the signalssensed from the substrate coil. This is possible due to the closeproximity of the substrate and the pattern generation tool, and theintegrated control logic electronics of the pattern generation tool.Alignment sensing accuracies smaller than 10 nm can be achieved in thismanner. The electrode pads of the substrate coil typically havedimensions of less than 2 mils on a side, and the probe point electrodesthat extend from the pattern generation tool to make contact with thesubstrat coil pads are typically less than 12 μm in diameter and height.A fluid pressure applied to the back side of the pattern generation toolmay be used to bring the surface of the pattern generation tool in nearcontact (proximity) or full contact with the surface of the substrateduring the alignment process.

Alternately, FIG. 28 b shows an alignment coil 495 that can be placed onthe substrate for alignment sensing by the pattern generation tool. Asignal is induced in one side 495 a of the coil by a corresponding coilin the pattern generation tool. This signal is then sensed at the otherend 495 b of the coil by a second coil in the pattern generation tool.Alignment is achieved in the same manner as discussed above.

Inexpensive piezoelectric motion control devices are commerciallyavailable from Burleigh Instruments, Inc. (Fisher, N.Y.) thatdemonstrate motion control capability of less than one. Angstrom; suchpiezoelectric precision motion has been demonstrated in the AFM (AtomicForce Microscope) equipment where motion control of less than oneAngstrom is required.

MDI Circuit Membrane Flat Panel Display

Much of the cost of manufacturing a flat panel display (withoutconsideration for yield loss) is the cost of the substrate on which itis fabricated. The substrat must meet requirements of flatness imposedby lithography tools and high temperatures imposed by variousfabrication. steps.

An MDI circuit membrane flat panel display is formed on a rigidoptically flat reusable substrate such as fused silica or fused quartz.Other substrates can be used to meet the requirements imposed by thefabrication processing steps. A release agent such as KBr or KBO₂ isdeposited onto the substrate and then a dielectric layer or layers aredeposited to seal the release agent deposited film (as described above).The release agent is required to have a working temperature higher thanthe highest temperature of the various fabrication process stepsemployed and be readily soluble in a solvent that is not detrimental tothe finished display device. As an example, KBr has a melting point of734° C. and is readily soluble in DI water.

The flat panel display is conventionally composed of rows and columns ofpixel elements which generate the gray shades or the RGB (Red, Green,Blue) colors of the display. The operation of the pixels results in thegeneration of the image on the display. The pixels of the display can bemade with conventional active matrix LCD technology. The preferredembodiment of the MDI circuit membrane display uses redundant circuitdevices at each pixel and the above referenced fine grain testingtechnology to correct for circuit fabrication defects.

Secondly, the KDI circuit membrane display makes use of electroluminescent phosphors, similar to the phosphors used in conventional TVand industrial monitors. The phosphors are applied after the completionof the fabrication of the circuitry of the display. The displaycircuitry is then used in the final manufacturing process steps to causethe selective deposition of the RGB phosphors to their respective RGBelectrodes.

Additionally, the multiple chip module interconnect circuit membranetechnology disclosed above also is employed to bond ICs at the edges ofthe display panel and at any desired location on the back of the panel.The ability to attach ICs is of significant importance in lessening thecomplexity of the design and manufacturing of the display. ICs bonded tothe back of the display do not interfere with or obscure the visualoperation of the phosphor based display, because the display is not backlit as is the case with LCD displays, but generates its ownluminescence.

FIG. 31 a shows the cross-section of a MDI circuit membrane display 700while still attached to a reusable quartz fabrication substrate 702 withintervening release agent 703. The control logic circuit 704-1, 704-2,704-3 for each RGB pixel has been fabricated from a-Si or polysiliconthin film transistors (TFTs), and electro-phoretic plating electrodes708-1, 708-2, 706-3 (pads) have been formed as a final metallizationprocess step over each RGB pixel logic circuit 704. The platingelectrodes 708-1, 708-2, 708-3, one for each RGB phosphor of a pixel,can be selectively addressed and a specific plating potential applied.At the end of each row and column of pixels are bonding pads 710 for ICs(die). The ICs provide control and memory logic for turning individualpixels on or off, therefore generating a display image. It should benoted that the control and/or memory logic 712, 713 (see FIG. 31 b)could also be fabricated as part of the MDI circuit membrane 700 just asthe individual pixel control circuits 704-1, 704-2, 704-3 have been.This is a design/manufacturing option, and would replace most or all ofthe bonded die. All fabrication processing steps of the MDI circuitmembrane are established techniques.

FIG. 31 b shows in cross section the MDI circuit membrane display 700attached to a support frame 718 and released from the reusablefabrication substrate. The support frame 718 is bonded to the MDIcircuit membrane 700 by an anodic method as one technique, and then theMDI circuit membrane 700 is scribed along the side of the fabricationsubstrate 702. This exposes the release agent 703 of FIG. 31 a. Thedisplay assembly is then placed in a solvent that activates the releaseagent 703, causing the display to be freed from the reusable fabricationsubstrate 702. Optionally, control and memory logic die 713 are attachedto the display backside. These IC 713 connect to pixels along row orcolumn interconnect metallization.

FIG. 31 c shows the display after electro-phoretic plating of theindividual RGB phosphors 708-1, 708-2, 708-3. Each RGB phosphor isplated separately by immersion of the display in a plating solution of aspecific phosphor, and then using the control and pixel logic 704-1,704-2, 704-3 of the display to select appropriate pixel elements (eitherRed or Green or Blue) and apply the desired plating electricalpotential.

The substrate used to fabricate the flat screen display could also be aMDI membrane with a semiconductor device layer. Using a semiconductorbased MDI membrane as the substrate to fabricate the flat screen displaywill not require any substantial difference in processing steps,however, due to the limited size of available silicon wafers (presently8 inches) and the circular shape, the use of a semiconductor wafer asthe starting substrate greatly limits the maximum size of the display.

Three Dimensional IC Structures

Three dimensional (3D) IC structures can be formed from MDI circuitmembranes. This is a novel capability of the MDI IC membrane. The lowstress dielectrics of silicon dioxide or silicon nitride can be formedto withstand working temperatures in excess of 400° C. This high workingtemperature capability allows the use of anodic or thermal (quartz toquartz or silicon to quartz) bonding procedures that are typically hightemperature processes. The dielectric circuit membranes are opticallytransparent and thin, allowing the circuit membranes to be aligned veryaccurately.prior to bonding. As shown in FIG. 32 a, the semiconductordevices 730-1, 730-2, 730-3 that are embedded in a first MDI circuitmembrane 732 can have interconnect metallization 736-1, 736-2, 736-3applied to both sides of the membrane 732 completing all intra-membraneinterconnections. Also shown are opposed semiconductor device gateelectrodes 742-1, 742-2, 742-3. FIG. 32 a shows the two MDI circuitmembranes or ICs 732, 746 prior to bonding. This allows the circuitmembranes to be coated as necessary for the forming of the anodic orthermal membrane-to-membrane bond 752 of circuit membrane 732 to thesecond membrane 746 which includes similarly semiconductor devices748-1, 748-2, 748-3, and interconnect metallization 750-1, 750-2, 750-3.After two membranes 732, 746 are bonded as shown in FIG. 32 b, onlyvertical interconnect metallization 754-1, 754-2 can be employed toroute signals from lower membrane layer 746.

MDI circuit membranes 732, 746 are aligned using established opticalalignment mark techniques and anodically or thermally bonded 752 to eachother. The circuit membranes 732, 746 that are anodically or thermallybonded are complete of all necessary semiconductor device processingsteps, and only require the completion of vertical interconnectmetallization 754-1, 754-2. The final interconnect metallization 754-1,754-2 is completed by etching vias 756-1, 756-2 through the last circuitmembrane 732 to be bonded to electrical contacts on the circuit membrane746 onto which it was bonded. Well known metal deposition and patterningprocess steps are applied. If additional circuit membrane ICs are to bebonded to a 3D IC structure, a dielectric coating is applied to the 3Dstructure to isolate any metallization as a necessary requirement to asubsequent bonding step.

Anodic bonding 752 is a glass-to-glass process. The MDI circuitmembranes 732, 746 are each provided with a 1 μm thick or greater finaldeposition of silicon dioxide to form the bond 752 prior to the anodicbonding process step. Anodic processing is well known and there arenumerous such techniques used in conventional glass plate manufacturing.

Thermal (fusion) bonding of silicon to quartz or glass substrates is anestablished technique. It can be used instead of anodic bonding methodsto bond MDI circuit membranes.

The ability to use optical data transmission as an interconnect means(instead of metallization interconnect) between MDI circuit membranelayers 732, 746 is a direct consequence of the method of fabricating the3D IC structure. Wave guides (not shown) between optical transceivercomponents can be formed from vias by bonding vias (openings)corresponding to the various optical transceiver devices on circuitmembranes. The normally high transmissive nature of the circuitmembranes and the short layer-to-layer distances between the circuitmembranes allow integrated optical semiconductor devices to provide thecommunication function through vias or directly through certaindielectric materials.

Integrated circuits formed on a MDI circuit membrane can be cut from thecircuit membrane in much the same manner as ICs or die are presently cutfrom a rigid wafer substrate in order to be packaged. This is possibledue to the net low stress of the circuit membrane. FIGS. 32 c and d showdie cut from a circuit membrane and bonded onto a rigid substrate suchas a silicon wafer, quartz, glass, metal, etc. In the case of a metalrigid substrate, dielectric and interconnect layers may be added toprovide a better bonding surface for the membrane IC or to provideinterconnection from bond pads on the IC to bond out pads at the edge ofthe metal substrate. The use of a metal substrate to attach the membraneIC provides clear advantages, if not an optimal means, for thermalcooling of the IC.

Once the circuit membrane substrate or wafer is cut into dice in themanner which is present practice, a pick and place bonding tool can pickup the die with slight vacuum pressure, align it over the substrate ontowhich it is to be bonded and bond it to the substrate. FIGS. 32 c and deach show stocked membrane die 764-a, 764-b, 764-c bonded to a rigidsubstrate. Alternately, the membrane die 764-a, 764-b, 764-c can bebonded with compressive, anodic or fusion (thermal) techniques to acircuit membrane (not shown), such as a MCM interconnect circuitmembrane.

FIG. 32 c shows a 3D circuit structure of membrane dice (ICs) 764-a,764-b, 764-c bonded by compression technique to a rigid substrate 770.This is accomplished by aligning metal pads 772-1, 1, 772-2, . . . ,772-k, . . . , 772-x on the rigid substrate 770 and metal pads 776-1,776-2, . . . , 776-w on the membrane die 764-a, . . . , 764-c and thenapplying temperature and pressure (as explained previously). Also shownare bonding-out pads 778-1, 778-2. The surfaces are bonded together bythe metal pads which also serve as circuit interconnect and thermalrelief paths.

FIG. 32 d shows a 3D circuit structure of membrane dice 764-a, 764-b,764-c bonded by anodic or fusion (thermal) techniques to a substrate770. The handling of the individual die 764-a, 764-b, 764-c can be donein a manner similar to that described above, and the bonding andinterconnect completion methods are also as described above for bondingthe circuit membranes together.

The benefits of such die assembly practice are dramatically increasedcircuit device densities and low thermal mass for efficient cooling. Thehandling equipment for making such a circuit assembly is presentlyavailable.

This disclosure is illustrative and not limiting; further modificationswill be apparent to one skilled in the art in light of this disclosureand the appended claims.

1. Circuitry comprising: a first integrated circuit having first activedevices; a second integrated circuit having second active devices, aplurality of metal interconnect layers, and at least one elasticdielectric layer at least in part between two of the plurality of metalinterconnect layers, wherein at least one of the plurality of metalinterconnect layers interconnects a plurality of the second activedevices underneath the at least one elastic dielectric layer; and aplurality of bonds formed between the first integrated circuit andsecond integrated circuit wherein the bonds provide a primary means ofadhesion between the first integrated circuit and the second integratedcircuit, wherein the at least one elastic dielectric layer issubstantially flexible, and wherein the at least one elastic dielectriclayer has a stress of at least one of less than about 8×10⁸ dynes/cm²and 2 to 100 times less than the fracture strength of the at least oneelastic dielectric layer.
 2. The device of claim 1, wherein said stressis tensile.
 3. The device of claim 1, wherein the second integratedcircuit is uniformly thinned throughout a full extent thereof.
 4. Thedevice of claim 3, wherein the thinned second integrated circuit issubstantially flexible while retaining its structural integrity.
 5. Thedevice of claim 3, wherein the thickness of the thinned secondintegrated circuit is less than about 50 microns.
 6. The device of claim1, further comprising vertical transmissions paths.
 7. The device ofclaim 6, wherein the vertical transmissions paths are electrical.
 8. Thedevice of claim 6, wherein the vertical transmissions paths are optical.9. The device of claim 1, further comprising interconnections connectingthe first integrated circuit and the second integrated circuit.
 10. Thedevice of claim 9, wherein the interconnections are verticalinterconnections that traverse at least one of the first integratedcircuit and the second integrated circuit.
 11. The device of claim 10,wherein at least one of the vertical interconnections are formed as partof one of the plurality of bonds.
 12. The device of claim 1, wherein theplurality of bonds joining the first integrated circuit and the secondintegrated circuit are compression bonds.
 13. The device of claim 1,wherein the at least one elastic dielectric layer is at least one of aninorganic dielectric material and an organic dielectric material. 14.The device of claim 13, wherein at least a major portion of theinorganic dielectric material is at least one of an oxide of silicon, anitride of silicon, silicon dioxide and silicon nitride.
 15. The deviceof claim 1, wherein the at least one elastic dielectric layer is capableof forming at least one of a flexible membrane, an elastic membrane anda free standing membrane.
 16. The device of claim 1, further comprisinga plurality of interconnect conductors formed as part of the at leastone elastic dielectric layer, wherein the interconnect conductors are atleast one of electrical and optical interconnect conductors.
 17. Thedevice of claim 1, wherein the at least one elastic dielectric layer isformed by at least one of Chemical Vapor Deposition and Plasma EnhancedChemical Vapor Deposition.
 18. The device of claim 1, wherein the atleast one elastic dielectric layer is formed by at least one of a RFenergy source and multiple RF energy sources.
 19. The device of claim 1,wherein the at least one elastic dielectric layer is formed at atemperature of about 400° C.
 20. The device of claim 1, wherein at leastone of the at least one elastic dielectric layer is caused to have atleast one of a tensile stress and a stress of at least one of about8×108 dynes/cm2 or less and 2 to 100 times less than the fracturestrength of the at least one elastic dielectric layer, the secondintegrated circuit is uniformly thinned throughout a full extentthereof, the second integrated circuit is uniformly thinned throughout afull extent thereof and substantially flexible while retaining itsstructural integrity, the second integrated circuit is uniformly thinnedthroughout a full extent thereof to a thickness of less than about 50microns, the bond joining the first integrated circuit and the secondintegrated circuit is a compression bond, the at least one elasticdielectric layer is at least one of an inorganic dielectric material andan organic dielectric material, at least a major portion of the at leastone elastic dielectric layer is at least one of an oxide of silicon, anitride of silicon, silicon dioxide and silicon nitride, the at leastone elastic dielectric layer is substantially flexible, the at least oneelastic dielectric layer is capable of forming at least one of aflexible membrane, an elastic membrane and a free standing membrane, theat least one elastic dielectric layer is formed by at least one ofChemical Vapor Deposition and Plasma Enhanced Chemical Vapor Deposition,the at least one elastic dielectric layer is formed by at least one of aRF energy source and multiple RF energy sources, and the at least oneelastic dielectric layer is formed at a temperature of about 400° C. 21.The device of claim 20, further comprising at least one of verticaltransmissions paths, electrical vertical transmissions paths, opticalvertical transmissions paths, interconnections connecting the firstintegrated circuit and the second integrated circuit, verticalinterconnections that traverse at least one of the first integratedcircuit and the second integrated circuit, at least one verticalinterconnection formed as part of one of the plurality bonds, and aplurality of interconnect conductors formed as part of the at least oneelastic dielectric layer, wherein the interconnect conductors are atleast one of electrical and optical interconnect conductors.